diff --git a/crates/clawhdf5-format/src/btree_v1.rs b/crates/clawhdf5-format/src/btree_v1.rs
index 7ae2af7..78b2329 100644
--- a/crates/clawhdf5-format/src/btree_v1.rs
+++ b/crates/clawhdf5-format/src/btree_v1.rs
@@ -158,7 +158,7 @@ impl BTreeV1Node {
 }
 
 /// Maximum recursion depth for B-tree traversal (malformed data protection).
-const MAX_BTREE_DEPTH: usize = 64;
+pub(crate) const MAX_BTREE_DEPTH: usize = 64;
 
 /// What a symbol table node takes with libhdf5's default group leaf K (4):
 /// its 8-byte header and 2K entries of 40 bytes (8-byte offsets). Hinted
diff --git a/crates/clawhdf5-format/src/group_v1.rs b/crates/clawhdf5-format/src/group_v1.rs
index f131acc..4a930dc 100644
--- a/crates/clawhdf5-format/src/group_v1.rs
+++ b/crates/clawhdf5-format/src/group_v1.rs
@@ -4,7 +4,7 @@
 use alloc::{string::String, vec::Vec};
 
 use crate::addr::checked_addr;
-use crate::btree_v1::collect_symbol_table_nodes_in;
+use crate::btree_v1::{BTreeV1Node, collect_symbol_table_nodes_in};
 use crate::error::FormatError;
 use crate::local_heap::LocalHeap;
 use crate::message_type::MessageType;
@@ -142,6 +142,95 @@ pub(crate) fn v1_group_entries<S: Storage + ?Sized>(
     }
 }
 
+/// The entry called `name` in a v1 group, looked up as libhdf5 looks it up
+/// (`H5G__stab_lookup`: `H5B_find` down the group's B-tree, then
+/// `H5G__node_found` in one symbol table node) instead of by reading every
+/// entry: at each node, the child whose key interval holds the name
+/// (left key < name <= right key, keys being names in the local heap,
+/// compared bytewise as `strcmp` does) is found by binary search. Reads
+/// O(depth) nodes and names, where listing reads the whole group.
+///
+/// `Ok(None)` when the search does not lead to the name. In a group whose
+/// B-tree is out of name order (damaged, or made by hand) that does not
+/// prove it absent, so callers then fall back to reading every entry;
+/// libhdf5 would report it missing.
+pub(crate) fn find_v1_entry<S: Storage + ?Sized>(
+    file_data: &S,
+    sym_table_msg: &SymbolTableMessage,
+    name: &str,
+    offset_size: u8,
+    length_size: u8,
+) -> Result<Option<GroupEntry>, FormatError> {
+    let heap = LocalHeap::parse_in(
+        file_data,
+        checked_addr(sym_table_msg.local_heap_address)?,
+        offset_size,
+        length_size,
+    )?;
+    // The keys and names are read from the heap's data segment one at a
+    // time: hint it (see `Storage::hint`).
+    file_data.hint(
+        heap.data_segment_address,
+        usize::try_from(heap.data_segment_size).map_or(1 << 20, |n| n.min(1 << 20)),
+    );
+    // As in listing: the heap's free list is checked before a name is used.
+    let mut heap_checked = false;
+    let mut name_at = |offset: u64| -> Result<String, FormatError> {
+        if !heap_checked {
+            heap.validate_free_list_in(file_data, length_size)?;
+            heap_checked = true;
+        }
+        heap.read_string_in(file_data, offset)
+    };
+    let want = name.as_bytes();
+    let mut address = sym_table_msg.btree_address;
+    for _ in 0..=crate::btree_v1::MAX_BTREE_DEPTH {
+        let node =
+            BTreeV1Node::parse_in(file_data, checked_addr(address)?, offset_size, length_size)?;
+        if node.node_type != 0 {
+            return Err(FormatError::InvalidBTreeNodeType(node.node_type));
+        }
+        // H5B_find's binary search with H5G__node_cmp3: go left when the
+        // name sorts at or before the left key, right when after the right
+        // key; otherwise this child holds it.
+        let (mut lo, mut hi) = (0usize, node.children.len());
+        let mut child = None;
+        while lo < hi {
+            let i = lo + (hi - lo) / 2;
+            let (Some(&left), Some(&right)) = (node.keys.get(i), node.keys.get(i + 1)) else {
+                return Ok(None);
+            };
+            if want <= name_at(left)?.as_bytes() {
+                hi = i;
+            } else if want > name_at(right)?.as_bytes() {
+                lo = i + 1;
+            } else {
+                child = Some(node.children[i]);
+                break;
+            }
+        }
+        let Some(child) = child else {
+            return Ok(None);
+        };
+        if node.node_level > 0 {
+            address = child;
+            continue;
+        }
+        let snod = SymbolTableNode::parse_in(file_data, checked_addr(child)?, offset_size)?;
+        for entry in &snod.entries {
+            if name_at(entry.link_name_offset)?.as_bytes() == want {
+                return Ok(Some(GroupEntry {
+                    name: String::from(name),
+                    object_header_address: entry.object_header_address,
+                    cache_type: entry.cache_type,
+                }));
+            }
+        }
+        return Ok(None);
+    }
+    Err(FormatError::NestingDepthExceeded)
+}
+
 /// Symbol table cache type for a soft link: the scratch pad's first four bytes
 /// are the local-heap offset of the link's target path, and the entry's object
 /// header address is undefined.
diff --git a/crates/clawhdf5-format/src/group_v2.rs b/crates/clawhdf5-format/src/group_v2.rs
index bc1a705..e902239 100644
--- a/crates/clawhdf5-format/src/group_v2.rs
+++ b/crates/clawhdf5-format/src/group_v2.rs
@@ -385,6 +385,27 @@ fn lookup_link<S: Storage + ?Sized>(
     length_size: u8,
 ) -> Result<Option<LinkTarget>, FormatError> {
     if is_v1_group(object_header) {
+        // Down the group's B-tree, as libhdf5 looks a name up; only when
+        // that does not find a hard link of that name is every entry read
+        // (a soft link, a group whose B-tree is out of order). A storage
+        // error (a read a restartable storage has not fetched yet) is
+        // returned as is: reading every entry would not get further.
+        if let Some(sym_msg) = object_header
+            .messages
+            .iter()
+            .find(|m| m.msg_type == MessageType::SymbolTable)
+        {
+            let stm = SymbolTableMessage::parse(&sym_msg.data, offset_size)?;
+            match group_v1::find_v1_entry(file_data, &stm, name, offset_size, length_size) {
+                Ok(Some(e)) if e.object_header_address != u64::MAX => {
+                    return Ok(Some(LinkTarget::Hard {
+                        object_header_address: e.object_header_address,
+                    }));
+                }
+                Err(e @ FormatError::Storage(_)) => return Err(e),
+                _ => {}
+            }
+        }
         let entries = resolve_group_entries(file_data, object_header, offset_size, length_size)?;
         if let Some(e) = entries
             .iter()
@@ -984,6 +1005,86 @@ mod tests {
         assert_eq!(values, vec![22.5, 23.1, 21.8]);
     }
 
+    /// `v1_groups_400.h5` from its superblock on (it has a user block).
+    fn v1_groups_400() -> (Vec<u8>, Superblock) {
+        let all: &[u8] = include_bytes!("../tests/fixtures/v1_groups_400.h5");
+        let data = all[signature::find_signature(all).unwrap()..].to_vec();
+        let sb = Superblock::parse(&data, 0).unwrap();
+        (data, sb)
+    }
+
+    /// Every child of a v1 group resolves by name, down the group's B-tree,
+    /// to the address the listing gives, reading a small part of what the
+    /// listing reads; a name the group does not hold is not found.
+    #[test]
+    fn v1_lookup_down_the_btree_agrees_with_the_listing() {
+        let (data, sb) = v1_groups_400();
+        let children = resolve_group_children(&data, &sb, sb.root_group_address).unwrap();
+        assert_eq!(children.len(), 401);
+        for c in &children {
+            let path = format!("/{}", c.name);
+            assert_eq!(
+                resolve_path_any(&data, &sb, &path).unwrap(),
+                c.object_header_address,
+                "{path}"
+            );
+        }
+        for missing in ["/g0400", "/a", "/g", "/g00000", "/zz", "/x0"] {
+            assert!(
+                matches!(
+                    resolve_path_any(&data, &sb, missing),
+                    Err(FormatError::PathNotFound(_))
+                ),
+                "{missing}"
+            );
+        }
+        let st = crate::storage::CountingStorage::new(data.clone());
+        resolve_group_children_in(&st, &sb, sb.root_group_address).unwrap();
+        let listing = st.bytes_read();
+        st.reset();
+        let last = children.last().unwrap();
+        assert_eq!(
+            resolve_path_any_in(&st, &sb, &format!("/{}", last.name)).unwrap(),
+            last.object_header_address
+        );
+        assert!(
+            st.bytes_read() * 8 < listing,
+            "lookup read {} bytes, listing {listing}",
+            st.bytes_read()
+        );
+    }
+
+    /// A v1 group whose B-tree is out of name order (a name changed in the
+    /// heap so that it sorts past every key) is still looked up by reading
+    /// every entry, as before the lookup went down the B-tree.
+    #[test]
+    fn v1_lookup_falls_back_when_the_btree_is_out_of_order() {
+        let (mut data, sb) = v1_groups_400();
+        let at: Vec<usize> = data
+            .windows(6)
+            .enumerate()
+            .filter(|(_, w)| *w == b"g0200\0")
+            .map(|(i, _)| i)
+            .collect();
+        assert_eq!(at.len(), 1, "one heap string");
+        data[at[0]] = b'~';
+        let children = resolve_group_children(&data, &sb, sb.root_group_address).unwrap();
+        let moved = children.iter().find(|c| c.name == "~0200").unwrap();
+        assert_eq!(
+            resolve_path_any(&data, &sb, "/~0200").unwrap(),
+            moved.object_header_address
+        );
+        assert!(resolve_path_any(&data, &sb, "/g0200").is_err());
+        for c in &children {
+            let path = format!("/{}", c.name);
+            assert_eq!(
+                resolve_path_any(&data, &sb, &path).unwrap(),
+                c.object_header_address,
+                "{path}"
+            );
+        }
+    }
+
     #[test]
     fn path_not_found_v2() {
         let file_data: &[u8] = include_bytes!("../tests/fixtures/v2_groups.h5");
diff --git a/crates/clawhdf5-wasm/tests/lazy.rs b/crates/clawhdf5-wasm/tests/lazy.rs
index 9c972e0..26e10af 100644
--- a/crates/clawhdf5-wasm/tests/lazy.rs
+++ b/crates/clawhdf5-wasm/tests/lazy.rs
@@ -475,11 +475,17 @@ fn corpus_files_read_the_same_lazily() {
     );
 }
 
-/// Passes and requests `list(path)` takes on a file opened lazily at
-/// `block`-byte blocks (the open not counted), checking the listing against
-/// the in-memory one.
-fn listing_cost(data: &[u8], path: &str, block: u64) -> (u64, u64) {
-    let want = Reader::open(data.to_vec()).unwrap().list(path).unwrap();
+/// What one call cost on a file opened lazily (the open not counted).
+#[derive(Debug, Clone, Copy)]
+struct Cost {
+    passes: u64,
+    requests: u64,
+    bytes: u64,
+}
+
+/// Open `data` lazily at `block`-byte blocks, run `op`, and return its
+/// result and what it cost (the open not counted).
+fn cost_of<T>(data: &[u8], block: u64, op: impl Fn(&Reader) -> T) -> (T, Cost) {
     let lazy = Lazy::open(
         data.to_vec(),
         LazyConfig {
@@ -489,14 +495,41 @@ fn listing_cost(data: &[u8], path: &str, block: u64) -> (u64, u64) {
     )
     .unwrap();
     let before = lazy.storage.stats();
-    assert_eq!(lazy.call(|r| r.list(path)).unwrap(), want);
+    let out = lazy.call(op);
     let after = lazy.storage.stats();
     (
-        after.passes - before.passes,
-        after.requests - before.requests,
+        out,
+        Cost {
+            passes: after.passes - before.passes,
+            requests: after.requests - before.requests,
+            bytes: after.bytes_fetched - before.bytes_fetched,
+        },
     )
 }
 
+/// Passes and requests `list(path)` takes on a file opened lazily at
+/// `block`-byte blocks (the open not counted), checking the listing against
+/// the in-memory one.
+fn listing_cost(data: &[u8], path: &str, block: u64) -> (u64, u64) {
+    let want = Reader::open(data.to_vec()).unwrap().list(path).unwrap();
+    let (got, cost) = cost_of(data, block, |r| r.list(path));
+    assert_eq!(got.unwrap(), want);
+    (cost.passes, cost.requests)
+}
+
+/// What reading the dataset at `path` whole takes on a file opened lazily
+/// at `block`-byte blocks (the open not counted), checking the values
+/// against the in-memory read.
+fn read_cost(data: &[u8], path: &str, block: u64) -> Cost {
+    let want = Reader::open(data.to_vec())
+        .unwrap()
+        .read(path, None)
+        .unwrap();
+    let (got, cost) = cost_of(data, block, |r| r.read(path, None));
+    assert_eq!(got.unwrap(), want, "{path}");
+    cost
+}
+
 /// An h5py file of `n` datasets of 256 `f32` each (`d0` ... ) in the root
 /// group, written with `libver`.
 fn h5py_many(dir: &Path, libver: &str, n: usize) -> Vec<u8> {
@@ -561,6 +594,38 @@ fn listing_a_large_group_takes_a_few_passes() {
     }
 }
 
+/// Opening one dataset of a large group looks its name up, not the whole
+/// group: in a v1 (symbol table) group down its B-tree as libhdf5 does, in
+/// a dense group down its name index. Reading one small dataset of 2000
+/// fetches a few blocks, where it used to read every symbol table node and
+/// name of a v1 group (529 requests, 333 kB at 512-byte blocks, before
+/// 2026-09-27).
+#[test]
+fn reading_one_dataset_of_a_large_group_fetches_a_few_blocks() {
+    if !python_available() {
+        assert!(
+            !std::env::var("CLAWHDF5_REQUIRE_INTEROP").is_ok_and(|v| v == "1"),
+            "CLAWHDF5_REQUIRE_INTEROP=1 but {} lacks h5py/netCDF4/numpy",
+            python()
+        );
+        eprintln!("skipping: {} lacks h5py/netCDF4/numpy", python());
+        return;
+    }
+    let dir = tempfile::tempdir().unwrap();
+    for (libver, most_passes, most_requests) in [("earliest", 7, 6), ("latest", 8, 9)] {
+        let data = h5py_many(dir.path(), libver, 2000);
+        for path in ["/d0", "/d1234", "/d1999"] {
+            let cost = read_cost(&data, path, 512);
+            eprintln!("h5py libver={libver}, 2000 children, read {path}: {cost:?}");
+            assert!(
+                cost.passes <= most_passes && cost.requests <= most_requests,
+                "{libver} {path}: {cost:?}"
+            );
+            assert!(cost.bytes <= 64 * 1024, "{libver} {path}: {cost:?}");
+        }
+    }
+}
+
 /// `CLAWHDF5_WASM_LIST_FILE=file.h5`: what listing the root group of that
 /// file costs lazily, and opening it and reading one dataset whole
 /// (`CLAWHDF5_WASM_READ`, by default the middle dataset of the listing),