diff --git a/environment.yml b/environment.yml
index 469dd68..ff473b6 100644
--- a/environment.yml
+++ b/environment.yml
@@ -4,7 +4,7 @@ channels:
   - r
   - bioconda
 dependencies:
-- python=3
+- python=3.6.7
 - pytest
 - pandas>=0.17.0
 - coverage
@@ -16,6 +16,7 @@ dependencies:
 - pysam
 - sphinx>=1.3.6
 - sphinx_rtd_theme
+- outrigger
 - pip:
     - graphlite
     - recommonmark==0.4.0