The systematics of the eastern North American isopods of the genus Asellus are revised, based on all available material (8918 specimens in 998 collections) and literature relating to eastern North American forms. Sixty-eight species (both epigean and hypogean) are recognized and discussed, five of which are newly described. Evidence is presented for synonymizing five nominal species of Asellus. A discussion is presented as to the generic status of the eastern North American isopods placed in the genus Asellus with the conclusion being reached that the proposed fragmentation of the genus is invalid (on the basis of the characters utilized). The existing species groups of troglobitic asellids have been modified and new species groups proposed for the epigean forms. The distributions of the various species groups and their individual species are given. The eastern North American isopods of the genus Asellus originated through multiple invasions of fresh water habitats primarily from early to middle Cenozoic during inundation of the coastal plains by a shallow marginal sea. Inland dispersal of these epigean ancestral stocks occurred by migration through then existing streams and rivers assisted most importantly, by the process of stream captures. Evolution of hypogean asellid stocks from epigean ancestral lineages is hypothesized. Subsequent patterns of speciation of troglobitic forms is discussed with consideration given to hypothetical barriers to dispersal. / Ph. D.