At the onset of adaptive differentiation caused by colonization of new environments, patterns of ancestral plasticity can influence trait expression, and thus determine which phenotypes are exposed to selection. This hypothesis can be evaluated in the postglacial adaptive radiation of threespine stickleback, Gasterosteus aculeatus, in northwestern North America. The very large oceanic populations that gave rise to the freshwater radiation are unlikely to have changed significantly since colonization was initiated, so the pattern of plasticity in oceanic fish can be used to infer the ancestral pattern. We demonstrate that courtship behaviour in oceanic sticklebacks is plastic and suggest that its expression is mediated by large cannibalistic foraging groups that attack and consume young in nests defended by males. We then demonstrate that, under laboratory conditions, males from two benthic (bottom-feeding) populations, where group cannibalism is common, zigzag less than males from a limnetic (plankton-feeding) population in which such groups do not form. The difference in courtship behaviour of these ecotypes parallels that seen in the field and the pattern of plasticity observed in oceanic fish. Ecotypic differences were also retained in laboratory-reared fish from one limnetic and one benthic population. Biting was a more common component of courtship in the laboratory than in the field, and males from freshwater populations bit more than those from the oceanic population, a difference that disappeared in laboratory-reared benthic and limnetic males. Our results suggest that ancestral plasticity may have influenced the subsequent evolution of some elements of courtship behaviour in the stickleback radiation. [Copyright &y& Elsevier]