Four species of Mallomys are recognised: M. rothschildi Thomas, 1898, with the subspecies M. r. weylandi Rothschild & Dollman, 1932 (syn. M. argentata Rothschild & Dollman, 1932); M. aroaensis (De Vis, 1907) with the subspecies M. a. hercules Thomas, 1912; M. istapantap n.sp., and M. gunung n.sp. Mallomys rothschildi is the smallest species. It is distinguished from the others by its relatively and often absolutely longer tail, short, diamond-shaped interparietal, and small cranial dimensions. It is found above about 1,500 m along the New Guinean Central Cordillera but is absent in the extreme southeast. It nests primarily in tree hollows. The subspecies M. r. weylandi is morphologically much more variable than any other Mallomys taxon. This variability may be the result of the absence of congeners within its range. Mallomys aroaensis is intermediate in size. Externally it differs from M. rothschildi except some individuals of M. r. weylandi in its lighter, grey colour, with long white-tipped guard hairs, and an ill-defined dorsal stripe. Cranially, its broad rostrum, inflated frontals, and great bizygomatic width relative to toothrow length distinguish it from all other species of Mallomys. It is found only in Papua New Guinea at altitudes from 1,100 m to about 2,450–3,850 m along the Central Cordillera, and to 3,600 m on the Huon Peninsula. It nests primarily in burrows. Mallomys istapantap n.sp. is the largest species, and appears to be restricted to subalpine grasslands and the upper montane forest fringe at 2,450–3,850 m in the east-central part of the New Guinean Central Cordillera. It is readily distinguished from all other species of Mallomys by its pale ears, and from M. rothschildi and M. aroaensis by its large size, short tail, large hypsodont molars, and numerous other cranial features (see diagnoses). It also nests in burrows. Mallomys gunung n.sp. resembles M. istapantap n.sp. externally, but lacks the pale ears of that species and differs markedly in cranial morphology (see diagnosis). Thus far it is known only from the western part of the Central Cordillera, from Mount Carstenz to Mount Wilhelmina, at altitudes of between 3,500 and 4,050 m.