Karyotypes of nine species ofRattusfrom the mainland of South Vietnam and one from Thailand are presented. Members of the subgenusRattus, includingRattus argentiventer, Rattus exiguus, Rattus exulans concolor, Rattus nitidus, Rattus norvegicus, Rattus rattus molliculusandRattus sladeni, had diploid numbers of 42 and very similar karyotypes.Rattus surifer finisandRattus moi, both in the subgenusLenothrix, had identical diploid numbers, 52, but dissimilar karyotypes. The karyotype ofRattus sabanus revertensfrom South Vietnam was similar to that ofRattus sabanusreported from Malaysia.