diff --git a/s2protocol/versions/__init__.py b/s2protocol/versions/__init__.py
index aaed865..2f7fb6a 100644
--- a/s2protocol/versions/__init__.py
+++ b/s2protocol/versions/__init__.py
@@ -1,47 +1,81 @@
-
+import logging
 import os
 import re
-import imp
+import importlib
 import sys
 
-
+PROTOCOLS=[]
 def _import_protocol(base_path, protocol_module_name):
     """
     Import a module from a base path, used to import protocol modules.
 
-    This implementation is derived from the __import__ example here:
-        https://docs.python.org/2/library/imp.html
+    This implementation is derived from the import_module example here:
+        https://docs.python.org/3/library/importlib.html#approximating-importlib-import-module
     """
 
+    requested_protocol = protocol_module_name
+    if protocol_module_name not in PROTOCOLS:
+        fallback_module_name = max(
+            PROTOCOLS,
+            key=protocol_version
+        )
+        logger = logging.getLogger(__name__)
+        logger.warning(
+            "Protocol %s not supported, using %s",
+            requested_protocol,
+            fallback_module_name,
+        )
+        protocol_module_name = fallback_module_name.split('.')[0]
+
+
     # Try to return the module if it's been loaded already
     try:
         return sys.modules[protocol_module_name]
     except KeyError:
         pass
 
-    # If any of the following calls raises an exception,
-    # there's a problem we can't handle -- let the caller handle it.
-    #
-    fp, pathname, description = imp.find_module(protocol_module_name, [base_path])
-    try:
-        return imp.load_module(protocol_module_name, fp, pathname, description)
-    finally:
-        # Since we may exit via an exception, close fp explicitly.
-        if fp:
-            fp.close()
-
+    # Otherwise try to load the module
+    spec = importlib.util.spec_from_file_location(
+        protocol_module_name,
+        os.path.join(base_path, protocol_module_name+'.py')
+    )
+    if spec is None or spec.loader is None:
+        raise ImportError(
+            f"Unable to load protocol module '{protocol_module_name}'"
+        )
+    module = importlib.util.module_from_spec(spec)
+
+    # If the module does not exist, raise an exception and let the
+    # caller handle it
+    if( not os.path.exists(spec.origin) ):
+        raise ImportError("No module named '{}'".format(protocol_module_name))
+
+    sys.modules[protocol_module_name] = module
+    spec.loader.exec_module(module)
+    return module
+
+def protocol_version(name):
+    m = re.search(r'(\d+)', name)
+    if not m:
+        raise ValueError(f"Invalid protocol name: {name}")
+    return int(m.group(1))
 
 def list_all(base_path=None):
     """
     Returns a list of the current protocol version file names in the versions module sorted by name.
     """
+    global PROTOCOLS
     if base_path is None:
         base_path = os.path.dirname(__file__)
     pattern = re.compile('.*protocol[0-9]+.py$')
-    files = [ f for f in os.listdir(base_path) \
-        if pattern.match(f) ]
-    files.sort()
-    return files
+    PROTOCOLS = [ f for f in os.listdir(base_path) \
+                  if pattern.match(f) ]
+
+    """
+    sort string containing integers is problematic.
+    """
+    PROTOCOLS.sort(key=protocol_version)
+    return PROTOCOLS
 
 
 def latest():