FOLLOWING a suggestion by Dr. H. Fairbairn, sleeping sickness officer, Tanganyika Territory, namely, that it might be possible to distinguish Trypanosoma brucei, gambiense and rhodesiense by the appearance, or the number, of chromosomes in each, the majority of the recognized chromosome-staining techniques were explored. Two methods gave good results, either using dried blood films from rats with very heavy infections, or probes of infective tsetses. The latter is a new method devised by Dr. E. Burtt, entomologist, at the Trypanosomiasis Research Station, Tinde, Tanganyika, in which the fly is caused to salivate upon a glass slide; I have modified the original method, in order to facilitate the handling of large numbers of tsetses, (a) The films are fixed with acetic alcohol1 and stained with Giemsa or haematoxylin. (b) The films are hydrolysed for 4–6 minutes in normal hydrochloric acid at 60° C. and then stained with Giemsa for 1–2 hours.