1) In a previous paper published in 1963, the author showed that in a population of Mus minutoides musculoides found in Bangui, Republic of Central Africa, there were individuals with 30, 31, 32, 33 and 34 chromosomes forming a polymorphic Robertsonian system. The present study shows that M. m. minutoides (Cape Province) and M. m. bellus (Tanganika) have 36 and 30 chromosomes respectively. The subspecies names are provisional since the difference in the chromosome complements of three forms is so great that interbreeding appears to be impossible even though they cannot be distinguished according to the methods of typological taxonomy. From the cytological standpoint we have three different species, respectively designated by the letters A (M. m. musculoides), B (M. m. bellus) and C (M. m. minutoides). Group bujo-triton: Three males of M. triton (Tanganika) were analysed and again found to form a polymorphic Robertsonian system with 20, 21 and 22 chromosomes. Two females have 21 chromosomes but their chromosome constitution does not agree with those of the males. Here we are dealing with another polymorphic system whose origin implicates complicated and numerous translocations. The sex chromosomes of M. m. minutoides are of the same type as those of M. musculus. In M. m. bellus they are like those of M. m. musculoides where it is probable that a translocation of both sex chromosomes onto a pair of autosomes has occurred (Matthey, 1957, 1958, 1963). The X of M. triton is big and sub-metacentric, the Y acrocentric and relatively small. he existence of at least two polymorphic systems (M. minutoides and M. triton) and of different chromosome complements found in the forms A, B and C seems to be linked with very active processes of transformation and partially explains the difficulties met by systematists in the study of pigmy-mice.