Trypanosoma brucei was first seen by David Bruce in 1894, in the blood of a cow in South Africa, and named in his honour in 1899. Trypanosomes seen in the blood of an Englishman in The Gambia in 1901 were named T. gambiense in 1902. Finally, in 1909, trypanosomes from the blood of an Englishman in Zambia ("Rhodesia") were named T. rhodesiense. Since then there has been continuous debate about the interrelationships of these three "species". Studies of the molecular biology of these trypanosomes, mainly analyses of their isoenzymes and deoxyribonucleic acid, now appear to have shown that T. "rhodesiense" cannot be distinguished from T. brucei brucei by any valid and consistent criterion, while T. "gambiense" probably does constitute a valid subspecies of T. brucei. There is still doubt whether populations of T. brucei are predominantly clonal or sexual. While some form of genetic exchange undoubtedly can occur in this species, its nature and frequency are unknown and there is evidence that the population structure of T. brucei is essentially clonal.