We examined seasonal and diel variation in the diet and prey selection (Chesson's α) of field-collected predatory stoneflies (Acroneuria abnormis). In the laboratory, we investigated how stoneflies feed on two morphologically and behaviorally distinct prey types (Baetis sp.andHydropsyche sp.) commonly found in the diet ofA. abnormis. OneA. abnormiswas presented with either: (1) 20Baetis sp., (2) 20Hydropsyche sp.added 30 hours prior to stonefly addition, thus having time to construct retreats, or (3) 20Hydropsyche sp.added just prior to stonefly addition, thus having no time to construct retreats. Stonefly behavior was videotaped at night during 30 minute trials in circular, plexiglass streams illuminated by infrared light. Numbers of prey encounters, attacks, and captures were recorded, and handling time was determined.