Abstract Even with the most standardized procedures yet available for in vivo trials, the intrastrain variations in the course of infection of Leishmania donovani in the hamster permit no easy assessment of variations among those tested by us from several parts of the world. Although more than a thousand hamsters have been used in the present evaluation, some of the strain differences observed may yet need further qualification. Upon intravascular introduction of similar numbers of parasites into hamsters, African, Mediterranean and Burmese strains of L. donovani appear to have distinct characteristics: early rates of accumulation of parasites, peak numbers in spleen and liver, and time to death all differ. Parasite counts in liver appear better to express the number differences than do those in the spleen under the experimental conditions observed. The African strains are most virulent and the Burmese least virulent, although all strains tested lead to the death of the hamster. Preliminary studies of some of the same strains of parasites in the mouse confirm the differences noted above except, of course, that deaths due to L. donovani in the mouse have not been observed.