Sika deer (Cervus nippon) in the Japanese archipelago are cervids known to have high ecological plasticity, which is reflected in latitudinal variation in their feeding habits. The northern populations are grazers that depend on dwarf bamboos, whereas the southern ones feed on browse and fruits. These differences in plant composition of the diet should affect the tooth wear patterns because the different plants have different concentrations and morphologies of phytoliths (microscopic silica bodies that precipitate in and around cells in many plants). We studied 2 mainland populations (Mt. Goyo and Oshika Peninsula) and 1 island population (Kinkazan Island) of sika deer from northern Japan to understand how dwarf bamboo (Sasa nipponica and Sasamorpha borealis) affects tooth wear on the mainland populations in comparison to the lawn grass (Zoysia japonica) consumed by the island population. Fruits are not considered in this study because only northern populations are examined. The combined analyzes of tooth mesowear and microwear permitted us to detect seasonal differences in the feeding habits of sika deer. The tooth wear pattern also reflected differences directly related to the type of plant ingested. The grass Zoysia japonica, available on Kinkazan Island, had a highly abrasive effect on sika deer enamel. The bamboo species S. nipponica and S. borealis, more abundant on the Mt. Goyo and Oshika Peninsula areas, respectively, were less abrasive than Z. japonica. Differences also were detected in the wear patterns produced by the 2 species of bamboo. Tooth mesowear suggests that S. nipponica is more abrasive than S. borealis, which was confirmed by the high phytolith content found in the plant, especially in the stem of S. nipponica, the part consumed by the sika deer in late winter.