We surveyed non-volant mammals in montane (1300 and 1400 m) and mossy forest (1500 and 1550 m) on Mount Cetaceo in the northern Sierra Madre of northeast Luzon in 2004 and 2005. We recorded a total of 12 species of mammals, including one shrew (Soricidae), seven murid rodents (Muridae), one deer (Cervidae), one wild pig (Suidae), one macaque (Cercopithecidae), and one civet (Viverridae). In this paper, we present the first ecological data on two recently discovered species endemic to northeast Luzon, Archboldomys musseri and Apomys sierrae. We captured only three species at 1300 m; captured five species at 1400 m; captured or inferred six species at 1500 m; and documented four species at 1550 m. Combined with an earlier study at 960 m (3 species), these results imply that species richness might increase from the lowlands to ca. 1500 m. The small number of sampling localities prevented strong statistical inference, but percent trap success with roasted coconut bait declined with increased elevation along our short transect, and trap success with live earthworm bait increased along the transect, so that overall trap success varied little along the transect. The most abundant species, A. sierrae, significantly preferred earthworm bait and was significantly more abundant in mossy forest than in montane forest. The percentage of diurnal captures along the transect increased with increased elevation, from a low of 0% at 1300 m to nearly 11% at 1550 m, even though no species showed significantly more diurnal than nocturnal/crepuscular activity. These patterns are similar to those on other mountain peaks on Luzon. The number of native species recorded on Mt. Cetaceo (7), plus one species recorded in the nearby lowlands, is similar to the number expected on the basis of the peak elevation of Mt. Cetaceo, supporting the hypothesis that the diversity of small mammals on a given mountain on Luzon is correlated with the elevation of the peak. Large mammals on Mt. Cetaceo are actively hunted; because they use both lowland and montane/mossy forest habitats, they may be threatened by the combination of agricultural expansion in the lowlands and unregulated hunting.