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A very large moth (10.0-12.0 cm. wingspan) with long, narrow forewings banded with light and dark grey-brown and a maroon hindwing with a blue-black blotch in the anal angle. Nearly impossible to mistake for any other Alberta moth. Very pale specimens from southern Alberta have been called Pachysphinx occidentalis. P. occidentalis is a much lighter tan moth with more extensive maroon shading on the hindwing. However, Alberta specimens appear to grade from pale to dark forms, and all Alberta Pachysphinx are treated here as modesta, pending further study.

Adult Big Poplar sphinx are nocturnal, and come readily to light. The large green larvae are reputed to be the largest insect in Alberta, based on weight. The larval stage extends from mid-July to September, and they overwinter in the ground as pupae.

Throughout much of North America, from the Gulf States north into the Boreal forest region across Canada. In Alberta, it occurs in cottonwood stands along the rivers of the plains (occidentalis ?) north into the boreal forest at least to Ft. McMurray, and west into the mountains at low elevations.