Nuclear and mitochondrial DNA polymorphism were studied in a number of natural populations of the Nile
tilapia Oreochromis niloticus (Cichlidae; Teleostei) from East Africa in order to determine the origin of a
recently discovered population from a warm water spring, the Lake Bogoria Hotel Spring, an affluent of the
Loboi Swamp. This population was initially considered to have been introduced from other sites within the
region. Its significant and unique genetic variability (high microsatellite and mtDNA polymorphism, highly
significant Fst values and the presence of private alleles) indicate however that it is an entirely new and
formerly unknown natural population that had escaped earlier studies of this species. This natural
population, that represents a significant genetic resource, is threatened by extensive human encroachment
of the Loboi Swamp.