Isozyme variation and fluctuating asymmetry at different ages of Atlantic salmonSalmo salarwere analyzed for hatchery and wild stocks. Significant age-related decrease of fluctuating asymmetry was detected in wild stocks but not in hatchery stocks. Natural selection may act against phenotype asymmetry, or some trait associated with asymmetry, in wild but not in hatchery populations. Heterozygosity did not vary significantly with fish age in most cases, suggesting the lack of association between heterozygosity and asymmetry.