The prickly sculpin, Cottus asper, is a polytypic species characteristically represented by very prickly, fresh-water spawning "inland" forms, and less prickly, brackish-water spawning "coastal" forms. Its widespread geographic distribution, pronounced phenotypic variability, and the effects of the contrasting environments in which it occurs are the subject of this investigation. A complete nomenclatural history and synonymy of the species for the period 1836-1936 is presented. Morphological evidence is interpreted in the light of field and laboratory studies of the migration and reproductive behavior. Differences found in distribution and intensity of body prickles (modified scales), geographical distribution, and migratory behavior, support the contention that the polytypy of "coastal" and "inland" forms of Cottus asper has a genetic basis. Other morphological evidence is equivocal in support of this interpretation. Differences in fin ray counts, e.g., pectoral rays, between "coastal" and "inland" forms are correlated positively with the presence or absence of salinity in the environment. However, it is not known whether such differences are the result of individual modifications induced by the local environment, or due to long term genetic fixation. Evidence from studies of closely-related species supports the interpretation that Cottus asper is a polytypic species which has become, and is in the process of becoming, modified into several characteristic genotypes. Evolution within this “asper species group" probably occurred in three stages, each correlated with past geologic history. During the first stage, perhaps in the Pliocene or Late Miocene, "coastal" and "inland" forms were derived from an ancestral marine cottid. The second stage, during the Pleistocene, was characterized by fragmentation of the gene pool of the "inland" form into a series of geographically isolated populations which have since evolved into valid species. The third stage, in Recent Time since the retreat of the Cordilleran glaciers, has occurred primarily within the northern representatives of the "coastal" form. At least two derivatives can now be distinguished within the "coastal" form. / Science, Faculty of / Zoology, Department of / Graduate