In his previous publications (Matthey, 1963–1966), the author has studied 26 individuals of the pigmy-mouse, Mus (Leggada) minutoides musculoides Temm., from different countries of Africa. This subspecies appears as a polymorphic Robertsonian system: the diploid numbers are equal to 22, 31, 32, 33, 34, while the number of major arms (N.F. = fundamental number) is constant, i.e., 36. The X chromosome is large and nearly metacentric, the Y large and submetacentric. It is demonstrated that these sex chromosomes are formed of an acrocentric X and Y (like those of the domestic mouse) translocated onto two homologous autosomes, which, at meiosis, furnish the pairing segments associated through a true chiasma. No variation of this pattern has been detected. In another species, Mus (Leggada) triton Th., the X chromosome is submetacentric (centromeric index, approximately 0.33), the Y short and acrocentric. In a sample from Bukavu (Congo), 11 females out of 21 show only one typical X, the other X is acrocentric and its length is equal to the length of the long arm of a normal X. The deletion of the short arm is also total or subtotal. No deletion occurs in the 18 males. Recently, five musculoides were obtained from the Ivory Coast. The three males, with diploid numbers of 34, 32, 32, are very similar to seven other specimens of the same country. Both females (2N =32) show only one normal metacentric X chromosome. The other X is submetacentric (I.C. = approximately 0.26) and appears to have lost the major part of one arm. Because of insufficient material, it is not possible to establish whether females homozygous for this deletion and males with the sex chromosome constitution Xd-Y occur in the populatio