diff --git a/requirements.txt b/requirements.txt
index 558c46bf07b3ba2ef5445049d08d01ab70cff5e7..e4494d30bbc0b9d28b6dd20e65452a60bdd3db81 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,2 +1,3 @@
 numpy
+cython
 simpletraj