The congeneric amphipods P. affinis and P. femorata are common and abundant detritivores in the Prudhoe Bay, Alaska, area. Their abundances and environmental data (depth, total organic C, bottom and surface temperatures, bottom and surface salinities and sediment grain size) were sampled at several stations in August of three consecutive years. An axis was determined through use of weighted discriminant analyses which would maximally separate the geographic distributions of the species. Temperature, depth and salinity were responsible for > 84% of the orientation of the axis. P. affinis was found in shallower, warmer and less saline waters than was P. femorata. Tolerance of these conditions allows P. affinis to invade nearshore areas which experience elevated temperatures, reduced salinites, and a large influx of detritus as the sea ice and rivers break up. P. femorata is less capable of invading this region, but is more abundant offshore than is P. affinis. The competitive advantages of P. femorata in the deeper, more marine-like areas are not known, but the literature suggests P. femorata may have a higher fecundity than does P. affinis.