The juveniles were released Wednesday into a creek at the University of California's James Reserve in the San Jacinto Mountains, about 100 miles east of Los Angeles, according to a statement from the U.S. Fish and Wildlife Service.
"We often think of endangered species as something exotic in far-away countries but we've got this one right here in Southern California," Adam Backlin, a U.S. Geological Survey ecologist who leads field monitoring of the species, said in the statement.
Efforts to reintroduce the yellow-legged frogs may help experts learn about how to help other amphibian species that are declining around the world, he added.
The frog, whose scientific name is Rana muscosa, is a 2- to 3-inch-long, mottled yellow hopper that once thrived in the high mountain streams of the Sierra Nevada and Southern California.
However, the tadpoles were tasty morsels for non-native trout that were used to stock the lakes. Studies also showed that livestock grazing and pesticide drift from Central Valley farms contributed to their decline.
Other threats may include a fungus, ash and debris from wildfires and damage to the frogs' egg sacs from recreational swimmers and stream crossers, experts said.
In 2003, federal officials found only about 200 were left in the San Gabriel, San Bernardino and San Jacinto mountains.
The California Fish and Game Commission voted last year to list the Sierra frogs as threatened and the southern frogs as endangered. The species also is a candidate for federal protection.
For several years, federal, state and private scientists have been trying to help the species recover by releasing hundreds of captive-bred frogs into the wild.
Those released into the James Reserve were bred at the San Diego Zoo's Institute for Conservation Research.
Natural populations of the frogs existed in the James Reserve as recently as the mid-1990s, reserve director Jennifer Gee said.
"The success of this effort will be used as a model for re-establishment of frogs in other streams within the frog's historic range," said Mike Giusti, senior environmental scientist with the California Department of Fish and Wildlife.
Native frog and toad species around the country and the world are in decline. Researchers say the reasons may include habitat loss, pesticide poisoning, disease and competition from invasive species.