The distribution of diagnostic allozymes at three loci among 33 populations of Aedes increpitus Dyar in the western United States revealed the existence of three genetically distinct population groups currently considered to be members of this taxon. On the basis of these biochemical characters and an analysis of morphological characters in larvae, pupae and adults, two new sibling species are described: Ae. clivis and Ae. washinoi. A description of biochemical characters of adults, and morphological characters of adults, pupae and larvae for each new species is presented. Aedes increpitus is redescribed on the basis of material from several western U. S. states. A discussion of the genetic structure of populations within the complex is presented, with special attention to the structure of populations as they occur in sympatry. Hybridization between Ae. increpitus and Ae. washinoi occurring in nature is described and the significance of interspecific hybridization as it relates to the taxonomic status of species is discussed.