diff --git a/src/mccode_antlr/translators/target.py b/src/mccode_antlr/translators/target.py
index b0f26d9..362bd32 100644
--- a/src/mccode_antlr/translators/target.py
+++ b/src/mccode_antlr/translators/target.py
@@ -216,8 +216,10 @@ def prefetch_data_files(self):
 
         Scans every component instance's string parameters.  For each literal
         string value that matches a registered data file (i.e. its registry path
-        includes a ``data/`` path component) the file is fetched into the local
-        Pooch cache so it is available before the compiled C instrument runs.
+        has a directory named ``data`` somewhere among its ancestors -- directly,
+        as in ``data/foo.dat``, or nested, as in ``data/ISIS_tables/TS2.imat``)
+        the file is fetched into the local Pooch cache so it is available before
+        the compiled C instrument runs.
         """
         from pathlib import Path
         if not self.registries:
@@ -240,13 +242,18 @@ def prefetch_data_files(self):
                     # Only consider exact basename matches inside a data/ directory.
                     # reg.known(name, strict=True) checks Path(x).name == name which is
                     # exact, but doesn't filter by directory.  We additionally require the
-                    # matched file to live under data/ to avoid fetching non-data assets.
+                    # matched file to live under a directory named data/ -- at any depth,
+                    # since McStas nests some data families a level deeper (data/ISIS_tables/,
+                    # data/Gas_tables/, ...) -- to avoid fetching non-data assets.
                     if not reg.known(name, strict=True):
                         continue
                     # Use the exact fullname (default exact=True) to avoid the loose
                     # substring fallback that raises on multiple matches.
                     fullname = reg.fullname(name)
-                    if fullname is None or f'data/{name}' not in str(fullname).replace('\\', '/'):
+                    if fullname is None:
+                        continue
+                    parts = str(fullname).replace('\\', '/').split('/')
+                    if 'data' not in parts[:-1]:
                         continue
                     cached = reg.path(name)
                     logger.info(
diff --git a/tests/translators/test_prefetch_data_files.py b/tests/translators/test_prefetch_data_files.py
new file mode 100644
index 0000000..fd5f321
--- /dev/null
+++ b/tests/translators/test_prefetch_data_files.py
@@ -0,0 +1,107 @@
+"""TargetVisitor.prefetch_data_files must find data files nested a level
+deeper than data/, e.g. McStas's data/ISIS_tables/ and data/Gas_tables/
+families (issue #329)."""
+from mccode_antlr import Flavor
+from mccode_antlr.translators.target import TargetVisitor
+
+
+class FakeValue:
+    def __init__(self, value):
+        self.is_str = True
+        self.has_value = True
+        self.value = value
+
+
+class FakeParam:
+    def __init__(self, value):
+        self.value = FakeValue(value)
+
+
+class FakeComponent:
+    def __init__(self, *values):
+        self.parameters = [FakeParam(v) for v in values]
+
+
+class FakeRegistry:
+    """Mimics the bits of RemoteRegistry/LocalRegistry that prefetch_data_files
+    relies on: known()/fullname() resolve a bare name against a fixed mapping
+    of registered files, and path() records what was actually fetched."""
+
+    def __init__(self, files: dict[str, str]):
+        # files: {bare_name: resolved registry path}
+        self._files = files
+        self.fetched = []
+
+    def known(self, name, ext=None, strict=False):
+        return name in self._files
+
+    def fullname(self, name, ext=None, exact=True):
+        return self._files.get(name)
+
+    def path(self, name, ext=None, exact=True):
+        self.fetched.append(name)
+        return f'/cache/{name}'
+
+
+class DummyInstr:
+    name = 'dummy'
+
+    def __init__(self, registries, components):
+        self.registries = registries
+        self.components = components
+
+    def verify_instance_parameters(self):
+        return None
+
+
+def _prefetch(registry_files, param_value):
+    reg = FakeRegistry(registry_files)
+    instr = DummyInstr([reg], [FakeComponent(param_value)])
+    visitor = TargetVisitor(instr, Flavor.MCSTAS)
+    visitor.prefetch_data_files()
+    return reg
+
+
+class TestNestedDataDirectory:
+    def test_file_nested_under_a_data_subdirectory_is_fetched(self):
+        """TS2.imat lives at mcstas-comps/data/ISIS_tables/TS2.imat -- one
+        directory deeper than the plain data/<name> case."""
+        reg = _prefetch(
+            {'TS2.imat': 'mcstas-comps/data/ISIS_tables/TS2.imat'},
+            '"TS2.imat"',
+        )
+        assert reg.fetched == ['TS2.imat']
+
+    def test_gas_tables_family_is_also_fetched(self):
+        reg = _prefetch(
+            {'He3.gas': 'mcstas-comps/data/Gas_tables/He3.gas'},
+            '"He3.gas"',
+        )
+        assert reg.fetched == ['He3.gas']
+
+    def test_file_directly_under_data_still_works(self):
+        """No regression on the original, simpler data/<name> layout."""
+        reg = _prefetch(
+            {'some_file.dat': 'mcstas-comps/data/some_file.dat'},
+            '"some_file.dat"',
+        )
+        assert reg.fetched == ['some_file.dat']
+
+    def test_file_outside_any_data_directory_is_not_fetched(self):
+        """A same-named non-data asset elsewhere in the registry must not be
+        pulled in just because it shares a basename with a string parameter."""
+        reg = _prefetch(
+            {'Something.comp': 'mcstas-comps/contrib/Something.comp'},
+            '"Something.comp"',
+        )
+        assert reg.fetched == []
+
+    def test_a_file_literally_named_data_does_not_spuriously_match(self):
+        """The check excludes the filename itself when looking for a `data`
+        ancestor, so a bare top-level file named `data.ext` isn't treated as
+        living under a directory called `data`."""
+        reg = _prefetch(
+            {'data.ext': 'mcstas-comps/data.ext'},
+            '"data.ext"',
+        )
+        assert reg.fetched == []