Like other striped swallowtails, it has a small orange eyespot near the lower angle of each hindwing; the eyespots can fool predators into attacking the rear of the butterfly instead of the head, giving the butterfly a chance to escape. Its appearance is similar to the western, and eastern tiger swallowtails, but has narrower black stripes and usually two tails on each hindwing (rather than only one). Most two-tails have a black "slit pupil" in the orange eyespot on each hindwing, never present in western tiger. It is also usually larger than similar swallowtails within its range, although some eastern tiger swallowtails are as big as any two-tailed. The wingspan ranges from 3 to 6.5 inches (7.6 to 16.5 cm), making it the largest swallowtail in western North America. As is the case with most swallowtails, females are larger and more brightly colored than males, having more blue and orange markings on the hindwings. It does not tend to hybridize with related species as do the western, eastern, and Canadian tiger swallowtails.
Can be seen from Guatemala, through Mexico, the western United States to southern Canada in southern British Columbia, Alberta and southwestern Saskatchewan. It typically lives near streams and in moist valleys but also in canyons and cities at lower elevations.