Observations by SCUBA in Georgian Bay and Algonquin Park, Ont., indicate fish have specific habits of diurnal, crepuscular, and nocturnal activity in freshwater lakes. Of 21 species investigated, 17 either schooled or aggregated in the daytime, but only one (Osmerus mordax) schooled at night. The number of fish apparent at night in shallow water was greater than in the daytime primarily because of the influx of offshore species and lack of cover for resting diurnal species. Color changes parallel those reported in tropical oceans; at night a barred or mottled pattern was assumed by some fish resting on the bottom and bold daytime horizontal patterns faded. The parallel evolution of these color patterns in fresh and salt water suggests a strong evolutionary pressure for prey animals to disguise body shape when at rest at night, and that bold color patterns are disadvantageous when swimming at night.