diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 93fbe10..50b201a 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -9,6 +9,8 @@ Changelog
 
 version 0.5.0-dev
 -----------------
++ Fix a bug where streams that were passed to gzip_ng_threaded.open where
+  closed.
 + Fix compatibility with Python 3.13
 
 version 0.4.0
diff --git a/src/zlib_ng/gzip_ng_threaded.py b/src/zlib_ng/gzip_ng_threaded.py
index f8c0820..5b8a9ff 100644
--- a/src/zlib_ng/gzip_ng_threaded.py
+++ b/src/zlib_ng/gzip_ng_threaded.py
@@ -78,16 +78,18 @@ def open(filename, mode="rb", compresslevel=gzip_ng._COMPRESS_LEVEL_TRADEOFF,
 def open_as_binary_stream(filename, open_mode):
     if isinstance(filename, (str, bytes)) or hasattr(filename, "__fspath__"):
         binary_file = builtins.open(filename, open_mode)
+        closefd = True
     elif hasattr(filename, "read") or hasattr(filename, "write"):
         binary_file = filename
+        closefd = False
     else:
         raise TypeError("filename must be a str or bytes object, or a file")
-    return binary_file
+    return binary_file, closefd
 
 
 class _ThreadedGzipReader(io.RawIOBase):
     def __init__(self, filename, queue_size=2, block_size=1024 * 1024):
-        self.raw = open_as_binary_stream(filename, "rb")
+        self.raw, self.closefd = open_as_binary_stream(filename, "rb")
         self.fileobj = zlib_ng._GzipReader(self.raw, buffersize=8 * block_size)
         self.pos = 0
         self.read_file = False
@@ -155,7 +157,8 @@ def close(self) -> None:
         self.running = False
         self.worker.join()
         self.fileobj.close()
-        self.raw.close()
+        if self.closefd:
+            self.raw.close()
         self._closed = True
 
     @property
@@ -246,7 +249,7 @@ def __init__(self,
         self._crc = 0
         self.running = False
         self._size = 0
-        self.raw = open_as_binary_stream(filename, mode)
+        self.raw, self.closefd = open_as_binary_stream(filename, mode)
         self._closed = False
         self._write_gzip_header()
         self.start()
@@ -334,7 +337,8 @@ def close(self) -> None:
         trailer = struct.pack("<II", self._crc, self._size & 0xFFFFFFFF)
         self.raw.write(trailer)
         self.raw.flush()
-        self.raw.close()
+        if self.closefd:
+            self.raw.close()
         self._closed = True
 
     @property
diff --git a/tests/test_gzip_ng_threaded.py b/tests/test_gzip_ng_threaded.py
index 871e3ae..1032a67 100644
--- a/tests/test_gzip_ng_threaded.py
+++ b/tests/test_gzip_ng_threaded.py
@@ -190,3 +190,22 @@ def test_gzip_ng_threaded_append_text_mode(tmp_path):
     with gzip.open(test_file, "rt") as f:
         contents = f.read()
     assert contents == "ABCD"
+
+
+def test_threaded_reader_does_not_close_stream():
+    test_stream = io.BytesIO()
+    test_stream.write(gzip.compress(b"thisisatest"))
+    test_stream.seek(0)
+    with gzip_ng_threaded.open(test_stream, "rb") as f:
+        text = f.read()
+    assert not test_stream.closed
+    assert text == b"thisisatest"
+
+
+def test_threaded_writer_does_not_close_stream():
+    test_stream = io.BytesIO()
+    with gzip_ng_threaded.open(test_stream, "wb") as f:
+        f.write(b"thisisatest")
+    assert not test_stream.closed
+    test_stream.seek(0)
+    assert gzip.decompress(test_stream.read()) == b"thisisatest"