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The variable oystercatcher (Haematopus unicolor), which has melanistic, pied and intermediate phases, was studied in the field in New Zealand from 1969 to 1972 inclusive. Parents and their progeny were colour-banded, and the development of plumage with age was checked for non-genetic variation. The parent-offspring data can be most simply explained by a genetic model involving a major gene (W) whose dominance is modified only in the heterozygous condition. In the model, the pied condition is homozygous dominant (WW), the melanistic condition is homozygous recessive (ww), and all intermediate-plumaged birds are heterozygous Ww. The wide range of plumage variability in the intermediate phase seems most likely explained by the action of polygenic modifiers acting cumulatively at a number of loci, and specifically on the Ww genotype. Gene frequencies and genotype frequencies are in Hardy-Weinberg equilibrium, indicating that selection does not operate against the heterozygote to form an isolating mechanism between the homozygous pied and melanistic phases. The three phenotypes are therefore best considered as colour phases of one species, H. unicolor.