We investigated local geographical variation in the plant composition and quality (crude protein) of the diet of the Japanese serow (Capricornis crispus) by analyzing rumen contents. We collected the rumen contents from 111 Japanese serows culled for pest control between January and March 1997 in Shizuoka Prefecture, central Japan. The composition of the diet, characterized by a high proportion of browse (65.2%), including green plant leaves (55.3%), confirmed that the Japanese serow is a typical browser, at least in winter. Although the composition of the diet varied geographically even within Shizuoka Prefecture (i.e. the northern populations included less green broad leaves, and more grasses and Sasa spp.), altitude did not affect the composition. Latitudinal variation in plant composition of the diet was explained by a shift in plant availability within vegetation zones from evergreen broad-leaf to deciduous broad-leaf forests. Lack of variation in the diet plant composition with changes in altitude may be explained by probably greater altitudinal than latitudinal variation in the territories of the Japanese serow, and by greater individual variation in food habits. Diet quality remained at a constant high level regardless of altitudinal and latitudinal variation in diet plant composition. This suggests that the Japanese serow maintains a high-quality diet by selective feeding, which is possible only when the serow is at low population density and in possession of a territory.