The Center for Biological Diversity filed its lawsuit Thursday in U.S. District Court.
The suit alleges the U.S. Fish and Wildlife Service has failed to protect the fox despite evidence that fewer than 50 individual foxes are known to survive.
Logging, trapping and other issues have contributed to the fox's decline, the suit claims, and climate change is projected to shrink the cool, high altitude habitat it needs to flourish.
The service said it had not reviewed the suit and could not comment.