This paper presents a chromosome analysis of five Mus bufotriton animals captured near Bujumbura (Burundi, Africa). The five Mus karyotypes fall into three chromosome entities: an "Ivory Coast-setulosus type" (2n = 36/FN = 36), a Robertsonian polymorphic system (2n = 30 or 32/FN = 35), and a "mattheyi type" (2n = 36/FN = 36). The "Ivory Coast-setulosus type" is represented by one female M. bufo specimen, the polymorphic system by two male and one female M. triton specimens, and the "mαttheyi type" by a male Mus specimen whose morphology closely resembles that of M. triton. The phylogenic relationships among these three entities are discussed with respect to taxonomy and the formerly described presumptive ancestral karyotype of African Mus. M. triton seems to present a special sex-determination mechanism; the three animals captured have only one sex chromosome (the X), which is morphologically identical in both sexes.