The morphology of a micro-organism, Cytœcetes microti, new genus, new species, which invades the granulocytes and occasionally the lymphocytes, is described. It occurs naturally in the vole, Microtus pennsylvanicus, is transmissible to white-footed mice, Peromyscus leucopus and Peromyscus eremicus, the common tame mouse, the white rat, but inoculations of guinea-pigs. domestic rabbits, a kitten and young chickens have resulted negatively. The only pathological effect attributable thus far to this organism is an increase in the number of granulocytes found in certain organs such as the spleen and adrenals, and degenerative changes in the individual cell invaded. Splenectomy performed soon after the microscopic disappearance of Cytœcetes from the circulating blood may result in its reappearance, when it may occur in a considerable proportion of the granulocytes, but nevertheless in smaller numbers than at the height of early infection. Overwhelming infections have in no instance followed splenectomy. The probable relationship of the organism to Ægyptianella pullorum (Carpano, 1928) is pointed out, but grounds for placing it in a separate genus are presented.