Field collections of Aedes atropalpus (Coquillett) were made from 22 sites in the United States, Canada, and El Salvador. Laboratory colonies were established from each site. Among the 22 strains, 4 distinct types were evident. These correspond to 4 subspecies: A. a. atropalpus (Coquillett), A. a. epactius Dyar & Knab, A. a. perichares Dyar, and A. a. nielseni O'Meara & Craig. Based on adult and larval morphology, 16 diagnostic characters were discovered. In addition, A. a. atropalpus differed from the other subspecies in several physiological and behavioral characteristics. Type-form females were autogenous and reluctant blood feeders, whereas females of the other 3 subspecies were predominantly anautogenous and avid blood feeders. Moreover, the rate of larval development was considerably slower in all type-form strains. Although each form possessed many distinctive characteristics, all 4 forms were interfertile. Both museum specimens and live material were examined in an effort to determine the distribution of each form. Each subspecies is confined to a limited portion of the range of the species. A. a. atropalpus was found along the east coast, in the Appalachian Mountains, and in the northern Great Lakes region in the United States and Canada. A. a. epactius was observed in the United States (Arkansas, Colorado, Missouri, Oklahoma, and Texas) and in Mexico, while A. a. perichares was found in El Salvador and Costa Rica. A. a. nielseni was obtained from Utah and Arizona. Several U. S. National Museum of Natural History specimens from El Salvador appeared to be intergrades between A. a. epactius and A. a. perichares. Additional sampling in southwestern United States and Central America would improve our knowledge of both the range of the 3 southern subspecies and the extent of their intergradation.