During a seach for mosquito larvae at Yaba, near Lagos, my attention was drawn to their complete absence from a swamp about a mile from the Medical Research Institute. The pools in the swamp appeared to be well suited for the growth of mosquito larvae, and I caught female mosquitos full of eggs in the adjoining fringe of bush. The absence of larvae was therefore evidently not due to any lack of fertile female mosquitos. An examination of the pools showed that they all contained small, active, surface-feeding fish,* belonging to the family Cyprinodontidæ, and it seemed probable that it was by their agency that the pools were kept free from larvae.