Relationship between fitness advantage of isozymic genotype and a state of genetic constitution in a stock was examined in cultured stocks of rainbow trout in Japan. A number of stocks exhibited a larger average standard length in heterozygouse individuals under a state of “heterozygote excess” stocks at the Idh-3, Pgm-2 and Sod, significantly. However, an additive effect of advantage of heterozygotes at the three loci could not be observed. It suggests a fitness advantage of individuals heterozygous at each locus in cultured stocks of rainbow trout. On the other hand, a few stocks exhibited a larger standard length in homozygous individuals under a state of “homozygote excess” at the three loci, suggesting a fitness advantage of homozygous individuals. The mechanism of such phenomena could be explained by the relationship between the state of genetic constitution and quantity of dereterious genes in such a stack. An appearance of a larger average standard length in heterozygotes than in homozygotes could be explained by the resulting of a low fitness in homozygote individuals including many deleterious genes in the stocks. An appearance of larger average standard length in homozygotes than in heterozygotes could be explained by the elimination of deleterious genes during the process of inbreeding.