Laboratory cultured amphipods, Amphithoe valida, were exposed to the water soluble fractions (WSF) of a No. 2 fuel oil for 6 days, and then transferred to clean sea water for one week. Survival and nesting behavior were observed daily, and the lipid contents were checked at the end of exposure and at the end of depuration. Survival of amphipods during exposure was high at all concentrations of W S F tested (0 to 25%). A delayed toxicity was observed; mortality was high in concentrations /> 15% WSF during depuration. The number of nests decreased with increasing concentration of WSF and length of exposure. Recovery of nest building activity in clean sea water was either lacking or small, indicating some damage to their nesting capability. The lipid contents of amphipods were close to the expected range (except at 25~ WSF) following the exposure, but dropped to about 1% following recovery compared with an expected value of 3.3%. Impairments of the chemosensory and locomotory systems may occur during exposure, which prevent amphipods from constructing nests for protection and food reserves, and eventually lead animals to use their stored energy of lipid for survival. Of the three biological parameters studied, nesting behavior and lipid content as a percentage of dry weight were better indicators of the sublethal stress of WSF than the response of survival; significant alterations of the former two parameters were found at the concentrations /> 5% WSF.Water, Air, and Soil Pollution 15 (1981) 185-195.