The karyotype and the spermatogenesis of Mus setulosus (G, Q, C bands and Ag-staining). - The G, Q and C banding patterns of Mus setulosus are described. In the male five Ag-NORs were detectable, located at the distal extremities of the chromosomes. The meiotic divisions of Mus setulosus were studied by silver-staining and C-banding. The X and Y chromosomes remain close to each other during leptotene, zygotene and pachytene; they do not establish and end-to-end or side-by-side association. During diplotene, diakinesis and metaphase I, they most often are separated. At pachytene the bivalents 5 and 7 lie near each other and form a heterochromatic vesicle. The r-RNA synthesis declines at the end of pachytene and stops during diplotene, diakinesis, metaphase I and II. During these four stages the centromeres are Ag-positive. A comparative analysis of G-bands revealed a great number of analogies between Mus musculus and M. setulosus, which confirms the hypothesis that M. setulosus originated from a Mus of the musculus group. Nevertheless the existence of non-homologous pairs and the differences in the distribution and the size of heterochromatic blocks show that the derivation of the karyotype of setulosus from that of musculus imply several complex rearrangements, the nature of which could not be determined despite the refinement of the actual techniques.