diff --git a/mitogen/__init__.py b/mitogen/__init__.py
index 26e051667..3ea3b8252 100644
--- a/mitogen/__init__.py
+++ b/mitogen/__init__.py
@@ -61,6 +61,7 @@
 
 import os
 _default_profiling = os.environ.get('MITOGEN_PROFILING') is not None
+_compression_level = int(os.environ.get('MITOGEN_COMPRESSION_LEVEL', '9'), 10)
 del os
 
 
diff --git a/mitogen/core.py b/mitogen/core.py
index 3bd23b35d..4ad4d590c 100644
--- a/mitogen/core.py
+++ b/mitogen/core.py
@@ -1334,13 +1334,14 @@ class Importer(object):
     if sys.version_info >= (3, 0):
         ALWAYS_BLACKLIST += ['cStringIO']
 
-    def __init__(self, router, context, core_src, whitelist=(), blacklist=()):
+    def __init__(self, router, context, core_src, whitelist=(), blacklist=(), compression_level=None):
         self._log = logging.getLogger('mitogen.importer')
         self._context = context
         self._present = {'mitogen': self.MITOGEN_PKG_CONTENT}
         self._lock = threading.Lock()
         self.whitelist = list(whitelist) or ['']
         self.blacklist = list(blacklist) + self.ALWAYS_BLACKLIST
+        self.compression_level = compression_level
 
         # Preserve copies of the original server-supplied whitelist/blacklist
         # for later use by children.
@@ -1356,7 +1357,7 @@ def __init__(self, router, context, core_src, whitelist=(), blacklist=()):
                 'mitogen.core',
                 None,
                 'x/mitogen/core.py',
-                zlib.compress(core_src, 9),
+                zlib.compress(core_src, self.compression_level),
                 [],
             )
         self._install_handler(router)
@@ -4155,6 +4156,7 @@ def _setup_importer(self):
                 core_src,
                 self.config.get('whitelist', ()),
                 self.config.get('blacklist', ()),
+                compression_level = self.config['compression_level'],
             )
 
         self.importer = importer
@@ -4185,6 +4187,7 @@ def _setup_globals(self):
         mitogen.context_id = self.config['context_id']
         mitogen.parent_ids = self.config['parent_ids'][:]
         mitogen.parent_id = mitogen.parent_ids[0]
+        mitogen._compression_level = self.config['compression_level']
 
     def _nullify_stdio(self):
         """
diff --git a/mitogen/master.py b/mitogen/master.py
index 03c897711..d27f324b9 100644
--- a/mitogen/master.py
+++ b/mitogen/master.py
@@ -1152,7 +1152,7 @@ def _build_tuple(self, fullname):
 
         if fullname == '__main__':
             source = self.neutralize_main(path, source)
-        compressed = mitogen.core.Blob(zlib.compress(source, 9))
+        compressed = mitogen.core.Blob(zlib.compress(source, mitogen._compression_level))
         related = [
             to_text(name)
             for name in self._finder.find_related(fullname)
diff --git a/mitogen/parent.py b/mitogen/parent.py
index 9d1a0ce3c..a9e6d97d5 100644
--- a/mitogen/parent.py
+++ b/mitogen/parent.py
@@ -57,6 +57,7 @@
 # Absolute imports for <2.5.
 select = __import__('select')
 
+import mitogen
 import mitogen.core
 from mitogen.core import b
 from mitogen.core import bytes_partition
@@ -696,7 +697,7 @@ class PartialZlib(object):
     def __init__(self, s):
         self.s = s
         if sys.version_info > (2, 5):
-            self._compressor = zlib.compressobj(9)
+            self._compressor = zlib.compressobj(mitogen._compression_level)
             self._out = self._compressor.compress(s)
             self._out += self._compressor.flush(zlib.Z_SYNC_FLUSH)
         else:
@@ -708,7 +709,7 @@ def append(self, s):
         final compressed output.
         """
         if self._compressor is None:
-            return zlib.compress(self.s + s, 9)
+            return zlib.compress(self.s + s, mitogen._compression_level)
         else:
             compressor = self._compressor.copy()
             out = self._out
@@ -1508,6 +1509,7 @@ def get_econtext_config(self):
             'whitelist': self._router.get_module_whitelist(),
             'blacklist': self._router.get_module_blacklist(),
             'max_message_size': self.options.max_message_size,
+            'compression_level': mitogen._compression_level,
             'version': mitogen.__version__,
         }