[4][5] Until 2001, the genus was believed to be monotypic, the single species being the tailed frog (Ascaphus truei Stejneger, 1899).

However, in that year, Nielson, Lohman, and Sullivan published evidence that promoted the Rocky Mountain tailed frog (Ascaphus montanus) from a subspecies to its own species.

This anatomical feature improves breeding success by minimizing loss of sperm in the turbulent, fast-flowing streams inhabited by this species.

The joints in amphicoelous vertebrae allow for significant lateral movement of the vertebral column, seen most clearly when fish use their tail to generate propulsive force.

An additional plesiomorphy is the presence of free ribs in adults, a characteristic only present in the basal group archaeobatrachia.

[10] Ascaphids lack the ability to vocalise, are small – around 2.5 to 5.0 cm (0.98 to 1.97 in) long – and are found in steep, fast-flowing streams in Montana, Idaho, Washington, Oregon, and northern California in the northwest United States, and southeastern British Columbia (Rocky Mountain Tailed Frog) and coastal BC (Coastal Tailed Frog).

Breeding season lasts from May through September, and females deposit their eggs in strings under rocks in fast-moving streams.

Unfortunately, movements and migrational habits in Ascaphus have not been well documented, preventing any conclusive statements on migratory behavior or philopatry from being made with confidence.

A wide variety of food items is taken, including both aquatic and terrestrial larval and adult insects, other arthropods (especially spiders), and snails.

The large, sucker-like mouth parts of the tadpoles are a second distinctive feature of the species, enabling survival in turbulent water unsuitable for other frogs.