Out of five black rats recently obtained in Sapporo, one specimen having 41 chromosomes is present. The complement shows one large metacentric chromosome. G- and C-banding analyses resulted in that the large metacentric element is produced through a new type Robertsonian fusion between each one of pair nos. 7 and 10. The remaining 4 rats have the ordinary 42 Asian type idiogram. Evidence seems to fairly support the view of the karyotype evolution through the Robertsonian fusion in the black rat.