diff --git a/importlib_metadata/__init__.py b/importlib_metadata/__init__.py
index c0f9e21d..93d4669c 100644
--- a/importlib_metadata/__init__.py
+++ b/importlib_metadata/__init__.py
@@ -15,13 +15,13 @@
 import contextlib
 import collections
 
+from ._collections import freezable_defaultdict
 from ._compat import (
     NullFinder,
+    Protocol,
     PyPy_repr,
     install,
-    Protocol,
 )
-
 from ._functools import method_cache
 from ._itertools import unique_everseen
 
@@ -710,8 +710,8 @@ class Lookup:
     def __init__(self, path: FastPath):
         base = os.path.basename(path.root).lower()
         base_is_egg = base.endswith(".egg")
-        self.infos = collections.defaultdict(list)
-        self.eggs = collections.defaultdict(list)
+        self.infos = freezable_defaultdict(list)
+        self.eggs = freezable_defaultdict(list)
 
         for child in path.children():
             low = child.lower()
@@ -725,6 +725,9 @@ def __init__(self, path: FastPath):
                 legacy_normalized = Prepared.legacy_normalize(name)
                 self.eggs[legacy_normalized].append(path.joinpath(child))
 
+        self.infos.freeze()
+        self.eggs.freeze()
+
     def search(self, prepared):
         infos = (
             self.infos[prepared.normalized]
@@ -736,7 +739,7 @@ def search(self, prepared):
             if prepared
             else itertools.chain.from_iterable(self.eggs.values())
         )
-        return list(itertools.chain(infos, eggs))
+        return itertools.chain(infos, eggs)
 
 
 class Prepared:
diff --git a/importlib_metadata/_collections.py b/importlib_metadata/_collections.py
new file mode 100644
index 00000000..f0357989
--- /dev/null
+++ b/importlib_metadata/_collections.py
@@ -0,0 +1,21 @@
+import collections
+
+
+class freezable_defaultdict(collections.defaultdict):
+    """
+    Mix-in to freeze a defaultdict.
+
+    >>> dd = freezable_defaultdict(list)
+    >>> dd[0].append('1')
+    >>> dd.freeze()
+    >>> dd[1]
+    []
+    >>> len(dd)
+    1
+    """
+
+    def __missing__(self, key):
+        return getattr(self, '_frozen', super().__missing__)(key)
+
+    def freeze(self):
+        self._frozen = lambda key: self.default_factory()
diff --git a/tests/test_integration.py b/tests/test_integration.py
index 11835135..00e9021a 100644
--- a/tests/test_integration.py
+++ b/tests/test_integration.py
@@ -7,6 +7,7 @@
     Distribution,
     MetadataPathFinder,
     _compat,
+    distributions,
     version,
 )
 
@@ -59,3 +60,16 @@ def test_search_dist_dirs(self):
         """
         res = MetadataPathFinder._search_paths('any-name', [])
         assert list(res) == []
+
+    def test_interleaved_discovery(self):
+        """
+        When the search is cached, it is
+        possible for searches to be interleaved, so make sure
+        those use-cases are safe.
+
+        Ref #293
+        """
+        dists = distributions()
+        next(dists)
+        version('importlib_metadata')
+        next(dists)