Differences in external body measurements and various skull characters between the two taxa, diardii and jalorensis, of Rattus rattus in Malaya are studied in order to clarify their systematic status. The results are expressed statistically and their significance tested. Highly significant differences were found between the two rats in tail length, length of hind foot and ear length. With the exception of interorbital breadth, differences in various skull characters between the two taxa are highly significant. Fore and hind limb bones are also measured and the differences in these between the two taxa are also significant. Skeletons of diardii and jalorensis rats are compared to study other skeletal differences. Structure of the os penis was studied in both taxa. It was found that immature males in both types of rats have only one ossified proximal bone while the distal bone is ossified on maturity. It is shown that the two taxa are reproductively isolated under natural conditions and have morphological differences of species rank (Dhaliwal, 1901a, b). Comparison with Ellerman's (1947) description of Rattus rattus shows that on the evidence of skull structure diardii is a subspecies of Rattus rattus. It is, therefore, suggested that diardii should be called Rattus rattus diardii (Jentink, 1879) while jalorensis should be called Rattus jalorensis (Bonhote, 1903). Evidence is given to show that there is no reason why the spelling of “diardii” should be replaced by “diardi” as has been done intermittently by Chasen (1933, 1940) and Harrison (1951, 1957).