F1 hybrids between the Asian type (2n = 42) and Ceylonese type black rats (2n = 40) invariably showed 41 chromosomes, while those between the Oceanian type (2n = 38) and the Ceylonese type usually showed 39 chromosomes. Twelve F2 hybrids (three litters) were obtained only from the latter combination. Among them, the karyotypes of nine rats were successfully analyzed. They were segregated into the Oceanian type (three rats) and the F1 type (six rats). No Ceylonese type was obtained. From the results of fertility testing in F1 hybrids between these three variants, the Oceanian type is assumed to be more closely related to the Ceylonese type than the Asian type. This coincided well with the results of karyotypic evolution of these geographic variants. Among the nine F2 hybrids examined, one female consistently showed 37 chromosomes with one X chromosome. This XO female appeared to be a normal female and to have been a sporadic natural occurrence.