In the grounds of the Medical Research Institute at Yaba, near Lagos, some 200 yards north-east of the laboratories there is a swamp, roughly circular, its diameter being about 20 yards. Until the present year, since first coming under observation in 1908, it has been definitely a pond, formerly used for watering cattle; small in the dry season, it sometimes assumed the dimensions of a lake in the rainy season. While it was a pond it harboured at least three different kinds of fish, and except for a few Anopheline larvae on rare occasions it was not a suitable breeding-place for mosquitos owing to the activities of the fish.