The activity patterns of animals, whether diurnal, crepuscular or nocturnal, are usually fixed endogenous rhythms, entrained by environmental Zeitgebers. Here we demonstrate that juvenile Atlantic salmon, Salmo salar, switch between diurnal and nocturnal foraging solely in response to environmental temperature, and independently of photoperiod and season. Above 10 degrees C juvenile Atlantic salmon fed predominantly during daylight, spending the night exposed in the water column but relatively quiescent. As temperature dropped below 10 degrees C they became increasingly nocturnal, hiding in refuges by day but emerging to feed at night. It has previously been shown that parallel physiological changes take place in the retinae of several species of salmonids: the quantity and composition of the visual pigments change so as to make the fish more dark adapted at low temperatures. As the fish were found to be far less aggressive by night than by day at all temperatures, the switch to nocturnal activity was also accompanied by a change in social structure. We suggest that this temperature-dependent strategy maximizes feeding efficiency in summer but reduces predation risk in winter.