This frog is a member of the mountain yellow-legged frog complex, which is comprised of two species: Rana muscosa and Rana sierrae. Both species are highly aquatic and are always found within a meter or two from the edge of water. Like Rana sierrae, Rana muscosa is yellowish or reddish brown from above, with black or brown spots or lichen-like markings. Toe tips are usually dusky. Underside of hind legs and sometimes entire belly is yellow or slightly orange, usually more opaque than in the Foothill Yellow-legged Frog, Rana boylii. Yellow coloration often extends forward to level of forelimbs. Dorsolateral folds present but frequently indistinct. The tadpoles are black or dark brown and are large (total length often exceeds 10 cm) and metamorphose in 1-4 years depending on the elevation.

Rana muscosa differs from Rana sierrae in having relatively longer legs. When a leg is folded against the body the tibio-tarsal joint typically extends beyond the external nares. The mating call of R. muscosa is significantly different from that of R. sierrae in that they lack transitions between pulsed and noted sounds. Both species call underwater. Males can be heard above water but only from a short distance away (<2 meters). The two species also differ in mitochondrial DNA. The mitochondrial DNA, male advertisement calls, and morphology datasets are geographically concordant (Vredenburg et al. 2007).
 
The southern California populations of the species were formally recognized as an Endangered distinct population segment as of July 2, 2002. For details please see the U.S. Federal Register at:

Southern California populations Endangered