A polymorphism in water-soluble muscle esterases in the freshwater cyprinid fish Alburnus alburnus, the bleak, is described. The variation may be attributed to three codominant alleles at a single locus. Strong distortions from Hardy–Weinberg expectation are noted in samples from all length groups of fish at all times of the year. All homozygotes and one heterozygote are generally in deficit, while the two common heterozygotes are present in excess. Analysis of the genotype frequencies on a basis of random mating indicates that the polymorphism is stable and close to equilibrium. There are significant associations between size class and the frequencies of alleles 1 and 2 and genotype 33.