Relatively few primates have been tested for susceptibility to Toxoplasma gondii and those tested have usually shown a high degree of resistance. Negative results were obtained in the following species, Cercopithecus patas and Papio sphynx by Levaditi and co-workers in 1929, Cercopithecus sabaeus and Papio doguera by Cowen and Wolf in 1945, Cynocephalus babuin by Levaditi and Schoen in 1933 and Macacus cynomolgus and Macacus sinicus by Nicolle and co-workers in 1909 and 1913. No symptoms were produced and no toxoplasmas found in the tissues by histological examination in the few instances where this was attempted. Only very small numbers of animals, usually one of each species, were used in these experiments. Macaca mulatta has been tested more extensively than the other species (Levaditi and co-workers, 1929; Sabin and Olitzky, 1937; Sabin and Ruchman, 1942; Cowen and Wolf, 1945). Levaditi and co-workers obtained totally negative results in a rhesus monkey inoculated intracerebrally.