The threespine stickleback,Gasterosteus aculeatus, is polymorphic for the arrangement of lateral bony plates. It is confirmed in this paper that four morphs (not three) should be distinguished in this species: low plated, low plated with a keel, partially plated and completely plated. A new model is proposed to explain the inheritance of these morphs which involves one major gene with three alleles displaying a dominance hierarchy withA (completely plated) dominant toa (low plated) which is dominant toa k (low plated with keel). The dominance of theA allele is modified to semidominance by a dominant alleleC at a second locus. This scheme explains all the results of relevant breeding experiments published so far. Field data also fulfill predictions derived from this model.