Little is known of the underlying behavioural mechanisms that allow invasive species to gain an ecological foothold in novel environments. We tested how the exotic Mozambique tilapia (Oreochromis mossambicus) might affect spawning success in groups of the native Australian eastern rainbowfish (Melanotaenia splendida splendida) under experimental tank conditions using the presence of either an isolated O. mossambicus male or a small breeding group of O. mossambicus males and females. Both egg production and the proportion of eggs fertilised in M. s. splendida were significantly reduced by over 70% and 30%, respectively, in the presence of O. mossambicus breeding groups, but were not significantly affected by the presence of only a single O. mossambicus male. These results suggest that the reproductive activities of O. mossambicus may severely disrupt the spawning success of this native rainbowfish and indicates that the ecological impacts of O. mossambicus might be far more serious than is currently thought.