Mastomys natalensis ("redeyed variant") proved to be as susceptible as suckling rats to infections with Trypanosoma (Trypanozoon) brucei gambiense (14 passages after isolation). The susceptibility of M. natalensis and rats was significantly higher than that of mice. After 21 further passages the T. (T.) b. gambiense strain was fully adapted to M. natalensis. Polymorphic trypanosome strains could be isolated from pigs and dogs in Liberia using M. natalensis as recipient rodents. Some of these strains were examined for their sensitivity to human plasma in the "blood incubation infectivity test" (BIIT) using M. natalensis as well. 6 strains were highly resistant resp. subresistant to human plasma and can be regarded as potentially infective (T. (T.) b. gambiense) for man. Referring to the investigations 1.) on the susceptibility, 2.) the isolation of trypanosome strains of the subgenus Trypanozoon from animal and 3.) the biological differentiation of these strains in the BIIT, M. natalensis can be regarded as suitable experimental animals for field and laboratory studies.