Activity rhythms and light intensity preferences of largemouth (Micropterus salmoides) and smallmouth (M. dolomieui) black basses were investigated in laboratory experiments designed to elucidate aspects of niche separation between these sympatric congeners. Both species exhibited crepuscular activity, but M. salmoides also showed a mid-day activity peak. M. dolomieui avoided bright light, and their peak activity periods occurred at the beginning and end of the dark periods. Activity of M. salmoides was much more depressed during the dark periods, and rose sharply at the onset of light periods. Activity records in constant light conditions showed these activity rhythms to be endogenous.