SummaryMorphological differences between the sexes in KeasNestor notabiliswere quantified from a sample of 86 sexed museum specimens, nine sexed zoo captives and 129 live, wild-caught birds. The results demonstrate that Kea are sexually dimorphic. Males are about 5% larger than females in linear measurements of body size and their upper bills are on average 12–14% longer, with a slightly larger radius of curvature. The dimorphism in bill size was statistically independent of the difference in overall body size, suggesting the possibility of intersexual differences in niche utilisation. Culmen length appears to be a useful means for distinguishing sexes in the field: our data indicate an optimum separation criterion of 43.9 mm for wild-caught individuals. Culmen length measurements from specimens originating in captivity were more variable. There was no evidence that sexual dimorphism increases with sexual maturity in this species.