As part of a study to quantify use of space and time by wild brown trout (Salmo trutta L.), we tested 12 trout (in four groups of three; four separate experiments lasting 8 days each ; 96 total fish-days of data) in an electronic shuttlebox (ichthyotron) under natural winter temperature and photoperiod conditions, to determine their diet patterns of locomotor activity . Activity was quantified as the number of photocell-monitored light-beam interruptions per hour as fish passed between chambers of the shuttlebox . Data from all four experiments, pooled and expressed as hourly percentages of 24-hour diel activity, yielded a bimodal (crepuscular) pattern, with a major peak at dawn and a lesser one around dusk . The fish were not fed during the experiments, so feeding schedules had no effect on these results . Another experiment with a single fish yielded results similar to the three-fish groups . Other experiments failed to demonstrate a free-running circadian rhythm in this species under constant darkness (DD), and this, along with an apparent lack of anticipation of light change in the activity pattern, suggests that the diel activity pattern observed in this species is exogenously controlled by either absolute light intensity or changing light intensity, or both . Field studies are underway to determine whether diel activity patterns of wild brown trout are the same under natural conditions .