Previous work has shown that juvenile Atlantic salmon, Salmo salar L, are predominantly nocturnal during winter (spending the day sheltering in streambed refuges) but become active 24 h a day in the summer. Observations of salmon in a semi-natural stream revealed how light, temperature and time of year determined these activity patterns; we also tested whether the life-history strategy of the fish affected diel activity, comparing fish that would migrate to sea the following spring with those that would be resident in fresh water for at least an additional year. Fish tended to hide at high light levels whenever the water was cold but were increasingly likely to emerge as the winter progressed. There were significant differences between the two groups of fish: the putative migrants sheltered more than the resident group in winter, but this trend was reversed in the spring. Reducing the risk of predation in winter may be one of the reasons for this seasonal change in behaviour.Copyright 1997 The Association for the Study of Animal BehaviourCopyright 1997The Association for the Study of Animal Behaviour.