diff --git a/README.rst b/README.rst
index 8611aab..c07837e 100644
--- a/README.rst
+++ b/README.rst
@@ -113,6 +113,8 @@ Changelog
 
 in-development
 ~~~~~~~~~~~~~~~~~~~
+* #138: The gzip default compression level is now 1 when no value is provided
+  by the calling function. The default used to be determined by the backend.
 * #135: xopen now uses zlib-ng when available and applicable.
 
 v1.8.0 (2023-11-03)
diff --git a/src/xopen/__init__.py b/src/xopen/__init__.py
index 287176e..3d00694 100644
--- a/src/xopen/__init__.py
+++ b/src/xopen/__init__.py
@@ -33,7 +33,6 @@
 import subprocess
 import tempfile
 import time
-import zlib
 from abc import ABC, abstractmethod
 from subprocess import Popen, PIPE, DEVNULL
 from typing import (
@@ -55,6 +54,8 @@
 # 128K buffer size also used by cat, pigz etc. It is faster than the 8K default.
 BUFFER_SIZE = max(io.DEFAULT_BUFFER_SIZE, 128 * 1024)
 
+XOPEN_DEFAULT_GZIP_COMPRESSION = 1
+
 igzip: Optional[ModuleType]
 isal_zlib: Optional[ModuleType]
 igzip_threaded: Optional[ModuleType]
@@ -1032,88 +1033,65 @@ def _open_zst(  # noqa: C901
     return f
 
 
-def _open_external_gzip_reader(
-    filename, mode, compresslevel, threads, **text_mode_kwargs
-):
-    assert mode in ("rt", "rb")
-    try:
-        return PipedPigzReader(filename, mode, threads=threads, **text_mode_kwargs)
-    except OSError:
-        return PipedGzipReader(filename, mode, **text_mode_kwargs)
-
-
-def _open_external_gzip_writer(
-    filename, mode, compresslevel, threads, **text_mode_kwargs
-):
-    assert mode in ("wt", "wb", "at", "ab")
-    try:
-        return PipedPigzWriter(
-            filename, mode, compresslevel, threads=threads, **text_mode_kwargs
-        )
-    except OSError:
-        return PipedGzipWriter(filename, mode, compresslevel, **text_mode_kwargs)
-
-
 def _open_gz(  # noqa: C901
     filename, mode: str, compresslevel, threads, **text_mode_kwargs
 ):
     assert mode in ("rt", "rb", "wt", "wb", "at", "ab")
-    # With threads == 0 igzip_threaded defers to igzip.open, but that is not
-    # desirable as a reproducible header is required.
-    if igzip_threaded and threads != 0:
-        try:
-            return igzip_threaded.open(  # type: ignore
-                filename,
-                mode,
-                isal_zlib.ISAL_DEFAULT_COMPRESSION  # type: ignore
-                if compresslevel is None
-                else compresslevel,
-                **text_mode_kwargs,
-                threads=1 if threads is None else threads,
-            )
-        except ValueError:  # Wrong compression level
-            pass
-    if gzip_ng_threaded and zlib_ng and threads != 0:
-        try:
-            if compresslevel is None:
-                level = zlib_ng.Z_DEFAULT_COMPRESSION
-            elif compresslevel == 1:
-                # zlib-ng level 1 is 50% bigger than zlib level 1.
-                # This will be wildly outside user ballpark expectations, so
-                # increase the level
-                level = 2
-            else:
-                level = compresslevel
-
-            return gzip_ng_threaded.open(
-                filename,
-                mode,
-                level,
-                **text_mode_kwargs,
-                threads=1 if threads is None else threads,
-            )
-        except zlib_ng.error:  # Bad compression level
-            pass
+    if compresslevel is None:
+        # Force the same compression level on every tool regardless of
+        # library defaults
+        compresslevel = XOPEN_DEFAULT_GZIP_COMPRESSION
+
     if threads != 0:
+        if igzip_threaded:
+            try:
+                return igzip_threaded.open(  # type: ignore
+                    filename,
+                    mode,
+                    compresslevel,
+                    **text_mode_kwargs,
+                    threads=1,
+                )
+            except ValueError:  # Wrong compression level
+                pass
+        if gzip_ng_threaded and zlib_ng:
+            try:
+                return gzip_ng_threaded.open(
+                    filename,
+                    mode,
+                    # zlib-ng level 1 is 50% bigger than zlib level 1.
+                    # This will be wildly outside user ballpark expectations, so
+                    # increase the level
+                    max(compresslevel, 2),
+                    **text_mode_kwargs,
+                    threads=threads or max(_available_cpu_count(), 4),
+                )
+            except zlib_ng.error:  # Bad compression level
+                pass
         try:
             if "r" in mode:
-                return _open_external_gzip_reader(
-                    filename, mode, compresslevel, threads, **text_mode_kwargs
-                )
+                try:
+                    return PipedPigzReader(
+                        filename, mode, threads=threads, **text_mode_kwargs
+                    )
+                except OSError:
+                    return PipedGzipReader(filename, mode, **text_mode_kwargs)
             else:
-                return _open_external_gzip_writer(
-                    filename, mode, compresslevel, threads, **text_mode_kwargs
-                )
+                try:
+                    return PipedPigzWriter(
+                        filename,
+                        mode,
+                        compresslevel,
+                        threads=threads,
+                        **text_mode_kwargs,
+                    )
+                except OSError:
+                    return PipedGzipWriter(
+                        filename, mode, compresslevel, **text_mode_kwargs
+                    )
         except OSError:
             pass  # We try without threads.
 
-    if "r" in mode:
-        if igzip is not None:
-            return igzip.open(filename, mode, **text_mode_kwargs)
-        elif gzip_ng is not None:
-            return gzip_ng.open(filename, mode, **text_mode_kwargs)
-        return gzip.open(filename, mode, **text_mode_kwargs)
-
     g = _open_reproducible_gzip(
         filename,
         mode=mode[0] + "b",
@@ -1124,13 +1102,14 @@ def _open_gz(  # noqa: C901
     return g
 
 
-def _open_reproducible_gzip(filename, mode, compresslevel):
+def _open_reproducible_gzip(filename, mode: str, compresslevel: int):
     """
     Open a gzip file for writing (without external processes)
     that has neither mtime nor the file name in the header
     (equivalent to gzip --no-name)
     """
     assert mode in ("rb", "wb", "ab")
+    assert compresslevel is not None
     # Neither gzip.open nor igzip.open have an mtime option, and they will
     # always write the file name, so we need to open the file separately
     # and pass it to gzip.GzipFile/igzip.IGzipFile.
@@ -1144,33 +1123,16 @@ def _open_reproducible_gzip(filename, mode, compresslevel):
     gzip_file = None
     if igzip is not None:
         try:
-            gzip_file = igzip.IGzipFile(
-                **kwargs,
-                compresslevel=isal_zlib.ISAL_DEFAULT_COMPRESSION
-                if compresslevel is None
-                else compresslevel,
-            )
+            gzip_file = igzip.IGzipFile(**kwargs, compresslevel=compresslevel)
         except ValueError:
             # Compression level not supported, move to built-in gzip.
             pass
     elif gzip_ng is not None:
-        if compresslevel == 1:
-            level = 2
-        elif compresslevel is None:
-            level = zlib_ng.Z_DEFAULT_COMPRESSION
-        else:
-            level = compresslevel
-        gzip_file = gzip_ng.GzipNGFile(**kwargs, compresslevel=level)
+        # Compression level should be at least 2 for zlib-ng to prevent very big files.
+        gzip_file = gzip_ng.GzipNGFile(**kwargs, compresslevel=max(compresslevel, 2))
 
     if gzip_file is None:
-        gzip_file = gzip.GzipFile(
-            **kwargs,
-            # Override gzip.open's default of 9 for consistency
-            # with command-line gzip.
-            compresslevel=zlib.Z_DEFAULT_COMPRESSION
-            if compresslevel is None
-            else compresslevel,
-        )
+        gzip_file = gzip.GzipFile(**kwargs, compresslevel=compresslevel)  # type: ignore
     # When (I)GzipFile is created with a fileobj instead of a filename,
     # the passed file object is not closed when (I)GzipFile.close()
     # is called. This forces it to be closed.