Among 20 hemograms (peripheric blood smears) carried out upon some adults male and female Giant Rats, (Cricetomys gambianus) captured in the Dakar region, two (10 p. 100) are infected by one Rickettsiale of the Ehrlichiae tribe and of the Cytoecetes genus Tyzzer, 1938. Polymorphonuclear neutrophil leukocytes and monocytes, more or less 1/25, and monocytes, more or less 1/40, of the systemic circulation are infected by "elementary bodies" (diameter : 0.1 to 0.3 mu) included in the cell protoplasma and bundled at one or two poles of the cell. They grow and become "initial bodies" (diameter : 1 to 1.5 mu). Sometimes dumbbell-shaped forms indicate an early particle division. They are included in a small but visible vacuole. The multiplication and the growth tend to the formation of an intravacuolar "morula" (diameter : 2.5 to 3.5 mu) that can deform the monocyte nucleus. Electron micrographs of thin sections of polynuclear neutrophil leukocytes infected by morula show that each element of the morula is surrounded by two membranes (internal and external). This prokaryotic element is carmine-coloured by staining technic of May-Grünwald and Giemsa. Hypochromia, anisochromia, anisocytosis, poikylocytosis, leukopenia (especially neutrophils), with hematopoiesis disorders (bone marrow lesions) and hematopedesis are observed. It is the first species of Cytoecetes infecting both monocytes and polynuclear neutrophil leukocytes of systemic circulation. The two Giant Rats, were also infected by some Grahamella and one massively by a spirochete of the genus Borrelia. Polyparasitism is probably the cause of the general and important blood disorders observed. This Cytoecetes is the second Ehrlichiae found in Senegal after Ehrlichia bovis. It is named Cytoecetes kamtchoulii n. sp., "Kamtchoouli" is the name of Cricetomys gambianus in Oualoff language of the Presqu'Ile du Cap Vert region. The ticks (Ixodidae) that live in the burrows of the rodent seem to be the vectors of this rickettsia.