March 18, 1998 – The Peninsular bighorn sheep was listed as endangered in March.

December 21, 1998 – The Center sued to force habitat protection for the species in December.

1999 – The U.S. Fish and Wildlife Service agreed to reconsider its decision to withhold habitat protection.

September 2000 – After the Center intervened, the city of Rancho Mirage refused to approve a massive development project in Peninsular bighorn sheep habitat. The development would have destroyed as much as 406 acres of sheep habitat.

October 25, 2000 – The Fish and Wildlife Service published a recovery plan for the sheep.s

February 1, 2001 – The Fish and Wildlife Service designated 844,897 acres of critical habitat for the sheep.

February 28, 2001 – In a landmark settlement with the U.S. Forest Service, the Center and allies won protections for more than 50 endangered species living in Southern California's four national forests, including the Peninsular bighorn sheep. The settlement instituted such sweeping changes as critical habitat protections, removing cattle from 226,026 acres of habitat, and closing two roads within the sheep’s lambing grounds.

2006 – When the Agua Caliente tribe and industry groups filed suit to vacate the U.S. Fish and Wildlife Service’s 2001 critical habitat designation, the Center and Sierra Club intervened. The final settlement, though, vacated 5 percent of Peninsular bighorn critical habitat (mostly on tribal lands) and remanded critical habitat designation to the Fish and Wildlife Service. The Service was required to publish new proposed critical habitat by late September 2007.

April 20, 2007 – The Center filed suit against the Army Corps of Engineers and the Fish and Wildlife Service for approving grading in Peninsular bighorn critical habitat and a plan that would have cut off an essential movement corridor for the bighorn to access the northern portion of their range.