Univariate and multivariate statistical analysis of 14 skull, dentary and dental characters and five external characters recorded from 78 specimens of Mus musculus indicated the occurrence of two distinct forms in southern and eastern Indonesia. The two forms were distinct from Mus musculus domesticus in Western Australia.

The more widespread form was attributed to M. m. castaneus which occurred on Bali, Lombok, Flores, Lembata, Alor, Kai, Banda, Neira, Timor and Roti islands. The other form all of which are new record for the reason from Kisar and Tanimbar Islands (which represent new distributional records for Mus musculus), differed consistently from M. m. castaneus. Descriminant Function Analysis (DFA) using a reduced set of five skull, dental and external characters correctly allocated 98% of individuals to their appropriate form; bulla length, C¹M³ length and M¹ length were important discriminants between the two forms.

Unlike local populations of Western Australian M. m. domesticus, which morphologically differed significantly, the island populations of the widespread M. m., castaneus did not appear to differ significantly.