(1) The individuals in the densely populated stock of brown trout, Salmo trutta L., living in Lake Lane and Stranda River, are small, mature at an early age, and have a relatively short life span. Such a demographic strategy appears favourable for populations having a large, but physically unstable spawning and nursery area, intense competition for food, and little vertebrate predation except for human exploitation. (2) For females, the age of sexual maturation appears largely dependent on growth-rate, size, and survival-rate of the fish. Males do not appear to have the advantage of a corresponding large body size. They mature when smaller and at more varying ages than the females, although the two sexes have almost identical growth-rates. (3) Sexually mature fish have higher condition coefficients than immature ones, which is probably an adaptation to ensure gonadal development and enhance the prospect of reproductive success. (4) Mortality appears to be largely dependent on the age when the fish achieve sexual maturation. Most individuals seem to die after their first spawning season. Females show a delay of one year in their sexual maturation relative to males. This adaptation gives protection to the females.