diff --git a/test/test_args.py b/test/test_args.py
index 5d39aec..3e0ab0e 100644
--- a/test/test_args.py
+++ b/test/test_args.py
@@ -156,3 +156,7 @@ def test_var_args():
 def test_targets():
   assert tftest.parse_args(targets=['one', 'two']) == sorted(
       ['-target=one', '-target=two'])
+
+
+def test_parallelism_arg():
+  assert tftest.parse_args(parallelism=10) == ['-parallelism', 10]
diff --git a/tftest.py b/tftest.py
index 1565cb6..a419503 100644
--- a/tftest.py
+++ b/tftest.py
@@ -90,6 +90,7 @@ def parse_args(init_vars=None, tf_vars=None, targets=None, **kw):
       cmd_args.append(flag)
 
   kv_flags = {
+      'parallelism': '-parallelism',
       'plugin_dir': '-plugin-dir',
       'state': '-state',
   }
@@ -286,7 +287,7 @@ def __init__(self, tfdir, basedir=None, binary='terraform', env=None,
     self.binary = binary
     self.tfdir = self._abspath(tfdir)
     self._env = env or {}
-    self.env = os.environ.copy()
+    self.env = {k: v for k, v in os.environ.items() if k != 'TF_LOG'}
     self._plan_formatter = lambda out: TerraformPlanOutput(json.loads(out))
     self._output_formatter = lambda out: TerraformValueDict(json.loads(out))
     self.enable_cache = enable_cache