Plan Vianney lake, situated in the Oisans massif (French Alps) at an altitude of 2250 m, has a fish fauna composed of brook trout, Salvelinus fontinalis (Mitchell), and Artic charr, Salvelinus alpinus (L.). The present study examines the feeding behaviour of S. fontinalis in 1993–94 but also gives some data on S. alpinus. The brook trout represented 80% of the fish fauna of the lake. Dietary analysis showed that this species is an opportunistic feeder and revealed similarities in the trophic niches of S. fontinalis and S. alpinus and a low level of potentially available food resources. The annual growth of S. fontinalis was short and the growth rate was slow compared with other populations. Under these extreme conditions, female fish required several summers to replenish their reserves of energy and were only in condition to spawn once every 2 or 3 years. Although the fecundity of the population was low, it was sufficient to maintain the species in the lake which is not exploited.