Surveys of small mammals on carefully selected mountains and mountain ranges on Luzon Island, Philippines, since 2000 have led to the discovery of seven previously unknown species of forest mice, Apomys, a remarkable radiation on just a portion of one island. On the basis of morphological and cytochrome (cyt) b DNA sequence data presented here, we propose a new subgenus, Megapomys, to include the large-bodied members of the genus, which form a monophyletic unit of relatively large mice (averaging ca. 65–110 g) with tails about as long as or slightly shorter than the length of the head and body; all of these species forage on the ground. Other members of the genus are assigned to the subgenus Apomys; they are smaller (ca. 18–41 g), have long tails, and usually or often forage above the ground surface. Members of the subgenus Megapomys include four previously recognized species (A. abrae, A. datae, A. gracilirostris, and A. sacobianus) and the seven new species described here (A. aurorae, A. banahao, A. brownorum, A. magnus, A minganensis, A. sierrae, and A. zambalensis). All occur in northern and central Luzon Island, with the exception of one species that occurs on Mindoro Island; none is present in southern Luzon. Each species can be distinguished both morphologically and genetically. Although there are few records of Megapomys below 500 m elevation, they are common above about 1000 m, and some species occur near the peaks of the highest mountains on Luzon (i.e., up to nearly 2900 m). On four mountain ranges, two species of the subgenus co-occur, one at lower and one at higher elevations, although there is usually some syntopic overlap. Sister-species usually occur allopatrically in different mountain ranges, with one possible exception. Some of these species occur in areas not previously known to support endemic mammals, indicating that these areas are previously unrecognized areas of mammalian endemism where further study is warranted.