Three electrophoretically and morphologically distinct populations previously referred to Phalanger orientalis are recognised within Papua New Guinea and the Solomon Islands: Phalanger orientalis orientalis from northern Papua New Guinea and some nearby offshore islands, Phalanger orientalis breviceps from the Bismarck Archipelago and Solomon Islands, and Phalanger intercastellanus from eastern and southern Papua New Guinea and the islands of Milne Bay. P. o. orientalis is genetically and geographically relatively uniform. P. o. breviceps may have been introduced by humans over most of its range, and it is extremely variable, even within island populations. P. intercastellanus shows considerable intraspecific geographic variation, and is genetically divergent from P. o. orientalis (Nei's unbiased distance of 0.216) and P. o. breviceps (Nei's unbiased distance of 0.171). Indeed, this divergence is so marked that the previously recognised taxa Phalanger carmelitae and Phalanger vestitus are apparently genetically closer to P. orientalis than to P. intercastellanus.