(Uploaded by Plazi for the Bat Literature Project) A detailed karyotypic analysis of two Japanese Vespertilio species,,V. superans and V. orientalis, was made on the basis of conventional data, G- and C-band patterns. No detectable karyotypic difference was found between the two species (2« = 38, FN = 54). By comparison of G-band patterns between Vespertilio species and Nyctalus lasiopterus (2n = 42, FN = 50), it was revealed that the four pairs (M-l, 2, 3 and 6) of six large M-autosome pairs found in Vespertilio are homologous to the four large biarmed pairs (nos. 1, 2, 3 and 8) in N. lasiopterus, and the remaining two large M-pairs (M-4 and 5) in Vespertilio have been formed by centric fusions of the four medium-sized A-pairs (nos. 5 and 9 for M-4, and nos. 6 and 11 for M-5). This indicates that Vespertilio evolved from a certain Nyctalus species, such as N. lasiopterus, having a somewhat derived karyotype (2» = 42, FN = 50).The similarity of the C-band pattern on the complements of Vespertilio species and N, lasiopterus suggests that the karyotypic alteration from N. lasiopterus-]ike form to Vespertilio might have accomplished without a significant change in amount of C-band material.