Rhythmic activity patterns are demonstrated for 15 species of fish in várzea lakes of the Solimões river by analyzing catch per hour-of-day data. Supplemented by visual observation, an array of six gillnets of equal length but different mesh size was used for collecting. The data, in general, show a bimodal pattern of activity. The two peaks of catch can be related to movement during twilight periods, while low catches can be ascribed to reduced activity during times close to mid-day or night Observed activity peaks correspond to local displacements of fish between feeding and shelter sites. In the case of piscivorous fishes, activity peaks could be explained by facility in prey localization, which is greatest during twilight periods when prey species are moving.