diff --git a/environment.3.5.yml b/environment.3.5.yml
index 48005951..ac652cd8 100644
--- a/environment.3.5.yml
+++ b/environment.3.5.yml
@@ -16,3 +16,4 @@ dependencies:
 - scipy=0.18.1
 - statsmodels=0.6.1
 - sympy=1.0=py35_0
+- pytest-xdist
diff --git a/environment.3.6.yml b/environment.3.6.yml
index 939d934f..bde96a0d 100644
--- a/environment.3.6.yml
+++ b/environment.3.6.yml
@@ -15,3 +15,4 @@ dependencies:
 - scipy=0.19.1
 - statsmodels=0.9.0
 - sympy=1.1.1
+- pytest-xdist=1.24.1
diff --git a/environment.3.7.1.yml b/environment.3.7.1.yml
index 7716d03c..af023289 100644
--- a/environment.3.7.1.yml
+++ b/environment.3.7.1.yml
@@ -16,3 +16,4 @@ dependencies:
 - scipy=1.1.0
 - statsmodels=0.9.0
 - sympy=1.3
+- pytest-xdist
diff --git a/environment.3.7.yml b/environment.3.7.yml
index 0907db2a..7f01e8c8 100644
--- a/environment.3.7.yml
+++ b/environment.3.7.yml
@@ -16,3 +16,4 @@ dependencies:
 - scipy=1.1.0
 - statsmodels=0.9.0
 - sympy=1.1.1
+- pytest-xdist
diff --git a/environment.nightly.yml b/environment.nightly.yml
index 9ee9678f..3a2dad55 100644
--- a/environment.nightly.yml
+++ b/environment.nightly.yml
@@ -14,3 +14,4 @@ dependencies:
 - scipy>=1.1.0
 - statsmodels>=0.9.0
 - sympy>=1.1.1
+- pytest-xdist