More than 100 mountain yellow-legged frogs died mysteriously in breeding tanks at the Fresno Chaffee Zoo, according to the Los Angeles Times.
Zoos in Fresno, Los Angeles and San Diego are all taking part in a breeding program.
Biologists had hoped to reintroduce the frogs, which suffered greatly after the 2009 wildfire in the Angeles National Forest.
For thousands of years, mountain yellow-legged frogs thrived in hundreds of streams cascading down the San Bernardino, San Gabriel and San Jacinto mountains.
Since the 1960s, the species has been decimated by fires, mudslides, pesticides, fungal infections, loss of habitat and the appetites of nonnative trout, bullfrogs and crayfish.
Fewer than 200 of them are believed to exist in the wild.
City News Service contributed to this report.