Effects of inbreeding on rainbow trout (Salmo gairdneri) were evaluated by a comparison of differences between inbred and outbred half-sib families reared as contemporaries during the 1st yr of life. Attention was focused on two levels of inbreeding expressed as inbreeding coefficients: F = 0.25, one generation of brother–sister matings, and F = 0.375, two generations of brother–sister matings. Although inbreeding at the F = 0.25 level in fall-spawning populations had no effect on egg hatchability, it significantly increased the frequency of crippled fry by 37.6% and significantly decreased feed conversion efficiency (5.6%), fry survival to 147 days (14.6%), and growth rate to 147 days (6%) and 364 days (23.2%). Effects of inbreeding at F = 0.375 were more pronounced; significant differences included increased frequency of crippled fry (191.5%) and decreased feed conversion efficiency (14.9%), fry survival to 147 days (29.7%), and growth rate to 147 day s (13.4%) and 364 days (33.5%). Application of the total effects of inbreeding on the number of fish remaining and the weight of fish remaining in a production lot at 1 yr of age indicated losses of 17.4 and 36.6% after one generation (F = 0.25) and 47.9 and 65.4% after two generations (F = 0.375) of brother–sister mating. Similar studies on a spring-spawning population produced similar results. General breeding methods that minimize the rate of inbreeding accumulation are discussed.