When two species co-occur, and the presence of one causes aggression in the second, this aggression may lower the effective birth rate of the second species by causing neglect of the brood. The equations expressing competition with aggressive neglect are of the form derived by Voltera for 2 species, one inhibiting the other more than itself. In a closed system equilibrium is monospecific but the species surviving depends on initial conditions. Ripley''s observations on Batjan show that Myzomela obscura and Nectarinia sericea do not inhabit quite identical niches, but in so far as aggressive neglect occurs some apparent indeterminateness in their occurrence is to be expected.