The diurnal and annual locomotory activity cycles of four brown trout confined in separate cages on the bed of Windermere and fed artificially from the lake surface are described. The annual activity curves were very similar, the fish being less active during the winter than in the summer with maximum activity occurring in June and again in August. The diurnal activity patterns were also very similar with a sharp rise in activity at dawn each day after an inactive night, the increased activity being maintained throughout the light period and falling rapidly at dusk.Two of the fish were fed daily at irregular times and two were fed automatically every 2 hr. The similarity of the activity patterns of these four fish and also the similarity to the patterns of unfed fish described in a previous paper suggests that light and not food is the prime stimulus to locomotory activity in brown trout.