Five cases of interbreeding between the Red-billed Gull Larus novaehollandiae scopulinus and the Black-billed Gull L. bulleri in the wild in New Zealand are described. Captive hybrids from the wild population were shown to be fertile. Comparison between two years' observations in the same colony suggests that interbreeding occurs only when isolated individuals with a strong readiness to mate are prevented from mating with conspecifics. Other cases of interbreeding in Laws spp. support this conclusion.