In the course of the in vitro cultivation of Trypanosoma brucei as midgut forms, immature metatrypomastigotes, and metacyclic forms, a phenomenon was observed that suggested possible existence of sexual forms consisting of two parasites of unequal size which were seen to come close to each other with their posterior ends apposed. The smaller parasite which was more active than the larger one travelled along the side of the latter so that its posterior end came to lie in the region of the kinetoplast of the larger one. At the latter site, the smaller parasite displayed vigorous twisting movements and later came to lie parallel to its partner. Electron-microscopic examination of the parasites revealed that the points of contact were the kinetoplast and the flagellar pocket. These observations point to the possible existence of sexual forms, and the transfer of genetic material in the 3 types of T. brucei cultured from the vector, G. m. morsitans.
