diff --git a/efficientdet/utils.py b/efficientdet/utils.py
index 340e63342..f8578a0bb 100644
--- a/efficientdet/utils.py
+++ b/efficientdet/utils.py
@@ -42,6 +42,8 @@ def activation_fn(features: tf.Tensor, act_type: Text):
     return tf.nn.relu(features)
   elif act_type == 'relu6':
     return tf.nn.relu6(features)
+  elif act_type == 'mish':
+    return features * tf.math.tanh(tf.math.softplus(features))
   else:
     raise ValueError('Unsupported act_type {}'.format(act_type))
 
diff --git a/efficientdet/utils_test.py b/efficientdet/utils_test.py
index 462b2ea69..75a3aedc2 100644
--- a/efficientdet/utils_test.py
+++ b/efficientdet/utils_test.py
@@ -114,6 +114,33 @@ def _model(inputs):
     self.assertIs(out.dtype, tf.float16)  # output should be float16.
 
 
+class ActivationTest(tf.test.TestCase):
+
+  def test_swish(self):
+    features = tf.constant([.5, 10])
+
+    result = utils.activation_fn(features, "swish")
+    expected = features * tf.sigmoid(features)
+    self.assertAllClose(result, expected)
+
+    result = utils.activation_fn(features, "swish_native")
+    self.assertAllClose(result, expected)
+
+  def test_relu(self):
+    features = tf.constant([.5, 10])
+    result = utils.activation_fn(features, "relu")
+    self.assertAllClose(result, [0.5, 10])
+
+  def test_relu6(self):
+    features = tf.constant([.5, 10])
+    result = utils.activation_fn(features, "relu6")
+    self.assertAllClose(result, [0.5, 6])
+
+  def test_mish(self):
+    features = tf.constant([.5, 10])
+    result = utils.activation_fn(features, "mish")
+    self.assertAllClose(result, [0.37524524, 10.0])
+
 if __name__ == '__main__':
   logging.set_verbosity(logging.WARNING)
   tf.disable_eager_execution()