A population of freshwater sculpin from Lake Washington differs from other species of the typically benthic genus Cottus in that individuals migrate nightly to the surface and midwaters. Individuals are phenotypically and biochemically similar to Cottus aleuticus, but have a longer, more terete body, a larger cephalic pore and canal system, shorter pelvic fins, and a higher frequency of individuals with seven branchiostegal rays per side.A close relationship between the Lake Washington population and C. aleuticus seems indicated because of their phenotypic and biochemical similarities and because of the existence of individuals in other Northwest lakes that appear intermediate in character between C. aleuticus and the Lake Washington population. The decision as to the specific status of the Lake Washington population is deferred until more is known of its degree of reproductive isolation from C. aleuticus and of the existence of these possibly intermediate populations.