SynopsisWe report the results of a field study testing influences of both density and changes over the diel cycle on food consumption and prey selection by brook charr, Salvelinus fontinalis. Charr density in replicate 35 m long sections of a New Hampshire stream was adjusted to either medium or high levels (relative to natural densities) . Diets of charr and the availability of drifting prey were then sampled every four hours for 24 hours .There were no significant diel changes in the weight of prey consumed by charr per four hours, though there was some indication of reduced feeding at night . Charr fed selectively on different prey taxa, showing most preference for cased caddis larvae . Several species of mayflies and stoneflies were selected more strongly during the day than at night . Charr fed selectively on larger prey during the daytime but showed no sizeselection at night . The density of charr had no significant effects on either their rate of food consumption or on selection for prey of different taxa or sizes .