Patterns of parasitaemia of rodent bloodstream trypanosomes were studied on agar with phase-contrast microscopy (agar technique). The parasites were three strains of Trypanosoma hrucei rhodesiense isolated from man in Botswana and Zambia. The parasitaemias in the normal and immunodepressed rats were oleomorphic. In all the three strains the patterns of parasitaemia were transiently enhanced by dexamethasone acetate but the enhancement exhibited when total body x-irradiation was applied was sustained. The superiority of the agar technique over the length-measurement methods in assessing pleomorphism is indicated. Light microscopy showed the possibilities of characterisation of animal and human species of African trypanosomiasis. This was based on the number, location and distribution of the biochemically stable linoprotein granules. Scanning electron microscopy illustrated details of the various forms of vascular trypanosomes of medical and veterinary importance. The existence, characteristics and relationship of the cylindrica 1 lv-shaped long-narrow trypanosome is discussed in contrast to the characteristics of the other major flat-looking forms. The appendages or filaments associated with the different forms are also exhibited. Stages of in vivo as well as in vitro phago-cvtosis of T.h. rhodesiense by peritoneal macrophages were also observed by scanning electron microscopy. Salicylhydroxamic acid in conjunction with glycerol was used to clear the blood of trypanosomes. The new populations were shown to be agranular in form and some of these were long-narrow. Observation on tsetse- transmitted infections and on the bloodstream culture forms showed that the first wave of parasites were agranular but no long-narrow forms could be identified. Forms from the peritoneal fluid in rats and ascites tumour fluid in mice were found to be pleomorphic. These included the giant forms of multinuclear, multi- flagellar and multikinetoplastic types. The roundbodied trypanosomes found in these fluids are related to those found in the vascular system while the multinuclear forms in the fluids are similar to those in the tissues. The tissue forms of Botswana strain of T.b. rhodesiense were studied by electron transmission microscopy. Some parasites located in the choroid plexus of SHAM-treated and untreated rats were discovered to be intracellular. The implication of this discovery in relation to life- cycle, pathology and chemotherapy is outlined.