Genetic variation at six enzyme loci was examined in named lines of Atlantic salmon in culture in Scotland, Twelve lines were examined in each of two years of study. Significant differences in allele frequencies were observed between the year classes of named lines and heterogeneity was present among lines in both years. Allele frequencies in farmed lines founded on fish taken from Scottish rivers or from Norwegian farmed populations differed significantly at most loci. In both cases allele frequencies differed from those in samples of wild fish taken from 18 rivers in Scotland. Most farmed lines based on the fish of single rivers also differed from wild fish in the specific rivers from which they had been derived. Genetic distances between farmed lines and their wild sources were of the same order as those between the salmon of different rivers in Scotland. No overall reduction in heterozygosity was detected in farmed lines. Most differences between the farmed lines and their wild sources appeared stochastic in origin.