Fry from three Vancouver Island chum salmon populations were reared under identical conditions from egg onward to determine if genetic divergence occures among populations that spawn in different seasons. Comparisons of genetic traits, such as incubation rate and external morphology, revealed significant differences between the autumn spawning stock and the two winter spawning stocks. At constant 6°C, simulated autumn spawning regime and simulated winter spawning regime incubation rates of winter stock embryos were more rapid than those of the autumn spawning population. However, genotype-environment interaction occurred at constant 10°C such that autumn stock embryos incubated more rapidly. Discriminant analysis of 10 morphometric features of the progeny separated groups corresponding to the three stocks. Environmental factors were important as the key characteristics of separation varied with temperature regime. Both genotype and genotype-environment interactions contribute to divergence among seasonally distinct spawning populations.