Most studies of the diel pattern of trout foraging have focused on examining gut fullness at different times of the day. We used radiotelemetry coupled with stomach content and macroinvertebrate drift sampling to compare the diel activity of Colorado River cutthroat troutOncorhynchus clarki pleuriticuswith variation of invertebrate drift and light intensity on six dates during summer 1993 in the North Fork Little Snake River, Wyoming. The percentage of trout active was greater during midday than at night (medians, 100% versus 40%) and active fish had fewer transmitter signal strength fluctuations per minute during crepuscular periods than during the remainder of the day (medians, 8.8 versus 12.3). Light intensity and daytime macroinvertebrate drift rate were significantly correlated with trout activity but 24-h drift rates were not. Stomach samples collected in the afternoon weighed significantly more than samples collected in the morning. Despite declines in daytime drift rate and stream discharge, the diel movement of trout also declined as summer progressed. Drift and diet significantly overlapped on all six dates, but selection occurred for certain taxa, especially terrestrial insects. We believe that Colorado River cutthroat trout in this stream forage primarily during the day in summer and that low light intensities prevented trout from exploiting the nocturnal increase in drifting invertebrates.