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Tailed frogs are small (40-50 mm snout-vent length) with a large head. They are the only members of family Ascaphidae and are considered to be "primitive" frogs, because they have nine presacral vertebrae, whereas most other frogs have 8, and they possess ribs, which have been lost in other groups of frogs. Adult males possess a short, cone-shaped "tail," which functions as a copulatory organ during mating. They are the only North American species that uses internal fertilization. Both tadpoles and adults possess morphological features functionally related to life in fast-flowing streams. Tadpoles have a flattened oral disc that produces a sucker-like mouth, which they use to attach to rocks in fast flowing water. The body is dorso-ventrally flattened, and the tail fin is low. Adults lack a tympanum (eardrum) and males do not call, which may be an adaptation to living within the constant noise of mountain streams. It is not known how mates find each other. While adults are mainly active at night, they can sometimes be found out and about in daytime, particularly in damp, cool settings.

Coastal species lay eggs later than the inland species, have smaller clutch sizes, shorter incubation periods and smaller hatching sizes than the Rocky Mountain Tailed Frog (Ascaphus montanus) (Karraker et al., 2006).

Tailed frogs are the longest-living frogs in North America, estimated to live 15 years or more. Development to adulthood is lengthy; tadpoles can take one to four years to metamorphose depending on geographical location (Bury and Adams 1999) and 7-8 years to reach sexual maturity.

The Coastal Tailed Frog and Rocky Mountain Tailed Frog (Ascaphus montanus) were formerly considered as single species (A. truei), but are now recognized as two species based on genetic and ecological differences (Nielson et al. 2001).