Ants are an important food resource for most of bear species. During the summer, Japanese black bears (Ursus thibetanus japonicus) use grasslands in the ∼60 km² Ashio area as an ant feeding site. We studied levels of myrmecophagy using GPS locations and activity sensor information along with direct observations of 2 bears during 2004 and 2005. We measured species composition, biomass, and nutrient contents of the ants and estimated use of ants through bear scat analysis. Both the number of ant species and biomass were higher in Ashio than in the adjacent forest areas. We recorded 15 ant species, 9 of which were fed on by the bears. Lasius flavus and L. hayashi were most abundant species and the species used by bears most often. Bears spent 7–8 hours/day feeding on ants. We estimated that they potentially ate 50,000–60,000 mg (dry weight)/day of ants, whose energy content was around 180–300 kcal/d, insufficient to meet their basal and field metabolic needs. Bears may have used ants for essential amino acids that they are unable to produce themselves. Assuming bears come to Ashio specifically for ants, these grasslands are valuable for bears at a time when vegetative food resources are limited.