Chromosomally the African Pigmy-Mice (Mus minutoides/musculoides)constitute a polymorphic system of Robertsonian type: 2n= 18–34, F.N. 36. The Leggadas of four populations originating from the Ivory Coast and the Central African Republic were studied: we report here the particular geographic distribution of the different karyotypes observed (the 2nvalues diminish in the West-East direction) and their frequencies. From this study we emphasize the hypothesis that speciation proceeds through successive steps which we can summarize in the following way: the first one is represented by the «initial polymorphism» of WALLACE (1959), the second one corresponds to a stage of chromosomal polymorphism and the last one is characterized by the establishment of a new species with a single diploid formula.