The western tiger swallowtail caterpillar is green, enlarged in the front, and marked near the head with four yellow dots and two yellowish eyespots with bluish centers. The "neck" is banded with black and yellow, and the body has several rows of tiny blue dots. The caterpillar turns brownish in the last stage prior to pupation. It can reach a maximum length of two inches.
As an adult, the butterfly is large, with a wingspan of 2¾ to 4 inches, and tailed. The upperside is bright yellow and edged with a thick, black border. Each side of the butterfly is marked with four wide, diagonal, nearly parallel stripes; the innermost stripe is the longest, while the outermost two are simply bars. The hindwing has several patches of blue and only one or two spots or crescents of orange. Underneath is similarly marked; the hindwing is edged with a continuous blue line followed by a row of yellow bars, and there is little orange.