In three different strains of brown trout a marked reduction in the number of circulating lymphocytes occurred in the blood of sexually mature male and female fish during the spawning season. This lymphocytopenia was closely correlated with elevated plasma cortisol levels in all three strains. It is well established that sexual maturation in salmonid fish is associated with an increase in susceptibility to disease: the present study provides strong evidence that this is mediated, at least in part, by a cortisol‐induced suppression of lymphoid activity. These findings are discussed in relation to possible physiological roles of cortisol during the spawning period and to the evolution of semelparity in salmonid fis