We have detected significant reduction in genetic variation at isozyme loci in a hatchery stock of west-slope cutthroat trout (Salmo clarki) in comparison to the wild stock from which it was derived 14 years earlier. This conclusion is supported by (1) a 57% reduction in the proportion of polymorphic loci, (2) a 29% reduction in the average number of alleles per locus, (3) a 21% reduction in the average heterozygosity per individual, and (4) significant changes in allelic frequencies between age-classes. This loss of variation is attributed to both a limited number of founders of the hatchery stock and the effects of genetic drift in the maintenance of the hatchery stock.