1. Lake Nabugabo, a small swampy lake, is separated from Lake Victoria by a relatively narrow sand-bar and swamp. The sand-bar is estimated to be about 4,000 years old; prior to that date the lake was a bay of Lake Victoria. A short description of the lake and its history is given.
2. The cichlid fishes are reviewed, mainly on the basis of new material collected by the Cambridge Nabugabo Biological Survey of 1962.
3. Six species of Haplochromis (five endemic to the lake) are now recorded; of these, two are new (Haplochromis simpsoni and H. venator; the latter was previously confused with H. pellegrini of Lake Victoria) and one (H. nubilus) is recorded for the first time.
4- Two other new records are: Hemihaplochromis multicolor and Astatoreochromis alluaudi. The latter species shows certain iiiteresting osteological differences when compared with specimens from Lakes Victoria and Edward.
5- The evolutionary history of the Nabugabo Haplochromis is discussed. The evidence strongly suggests that the endemic species were derived from Lake Victoria species similar to, if not identical with species still extant in Lake Victoria. The significance of this rapid speciation in understanding speciation in Lake Victoria Haplochromis is considered.