The Black Swallowtail has a wingspan of 3 1/4 inches to 4 1/4 inches (8 to 11 centimeters). The upper surface of the wings is mostly black. On the inner edge of hindwing is a black spot centered in larger orange spot. A male of this species has a yellow band near edge of wings; a female has row of yellow spots. The hindwing of the female has an iridescent blue band.
In the Southwest USA, yellow forms predominate in the subspecies Papilio polyxenes coloro.
After mating, small, yellow eggs are laid, typically on garden plants from the carrot family, including dill, fennel and parsley.They are also found eating bishop's weed. First instar larvae grow to about 1.5 cm (0.6 inch) long, resemble bird droppings and are dark purple-brown with a white band in the middle and have spikes, with a light brown-orange ring at the base of each of the spikes in the dark region (spikes are white on the white band). Later instars grow to about 5 cm (2 inches) and are yellow-white and black banded with yellow spots around every second black band. They have short, black spikes around some of the black bands, although these tend to disappear as the larva nears pupation.
The Black Swallowtail Caterpillar has an orange "forked gland", called the osmeterium. When in danger the osmeterium, which looks like a snake's tongue, everts and releases a foul smell to repel predators.
The Black Swallowtail pupae may be green or brown, but not depending on it's surroundings or what it has pupated on. The color of the chrysalis is determined by a local genetic balance which ensures that majority of pupae will blend in.
Hosts of P. Polyxenes include Parsley, Dill, Fennel, Carrot, Queen-Anne's Lace, Parsnip, and other relatives.