In this paper we discuss the contribution of cytogenetics to the systematics of Arvicanthis in East Africa, by reviewing all the known chromosomal cytotypes of the genus in the area. We also provide G- and C-banding comparisons for two recently described karyotypes, provisionally named ANI-5 (2n 1⁄4 56, NFa 1⁄4 62) and ANI-6 (2n 1⁄4 60, NFa 1⁄4 72). This, therefore, brings the total number of known cytotypes in this area to 10. Five of these correspond to the species recognized by the latest rodent checklist, i.e. A. nairobae (2n 1⁄4 62, NFa 1⁄4 78), A. neumanni (2n 1⁄4 52–53, NFa 1⁄4 62), A. blicki (2n 1⁄4 48, NFa 1⁄4 62), A. abyssinicus (2n 1⁄4 62, NFa 1⁄4 64) and A. niloticus (2n 1⁄4 62, NFa 1⁄4 60–62). The taxonomic status of the remaining five cytotypes (A. cf. somalicus, 2n 1⁄4 62 NFa 1⁄4 62–63; ANI-5, 2n 1⁄4 56, NFa 1⁄4 62; ANI-6/6a 2n 1⁄4 60, NFa 1⁄4 72/76; ANI-7, 2n 1⁄4 56, NFa 1⁄4 78; and ANI-8, 2n 1⁄4 44, NF 1⁄4 72) is discussed. Finally, we reconstruct the phylogenetic relationships among all the known karyotypes on the basis of banding data available for the genus in Africa and show the occurrence of two main clades, each characterized by different types of chromosomal rearrangements. The times of the cladogenetic events, inferred by a molecular clock, indicate that karyotype evolution has accomplished almost all the dichotomic events from the end of the Miocene to the present day. The discovery of a large chromosomal differentiation between populations showing low genetic distances and intrapopulation chromosomal polymorphism suggests that the process of chromosomal differentiation in Arvicanthis is still ongoing and may possibly be responsible for speciation