Aminopeptidase-I allozymes, which are products of the Lap locus in the marine mussel, Mytilus edulis, differ in their catalytic efficiencies. These biochemical differences result in genotype-specific rates of change in the free amino acid pool, that is, cell volume regulation, when mussels are subjected to changes in salinity. A high degree of dominance was found among genotypes for these biochemical and physiological phenotypes. Selection models that incorporate dominance adequately predict observed genotypic properties at the Lap locus among natural populations that exhibit clinical allele frequency. This suggests that a high degree of dominance for fitness must also occur at this locus in natural populations. These results provide additional evidence that the maintenance of an allele frequency cline is operating by natural selection at the Lap locus.