To examine the relationship between the annual variation in agricultural crop consumption by bears and the fluctuation in acorn production from 1994–1997, we investigated the diet of Hokkaido brown bears (Ursus arctos yesoensis) in the Tokyo University Forest in central Hokkaido, Japan, using scat content analysis, and estimated Quercus crispula acorn production using seed traps. Scat analyses revealed seasonal changes in the diet of bears; the dominant food items were forbs with small numbers of ants in early summer, crops in late summer, and berries and Q. crispula acorns in autumn. Acorn production fluctuated annually throughout the study and was highest in 1994 and lowest in 1995. From September through November 1995, the frequency of occurrence of acorns was lower, and that of crops was higher, than in the other years. We found a negative correlation between the difference in the same month of the previous year in percent volume of crops in the diet and that in acorn production in September through October, 1994–1997. In years of low acorn production, brown bears used crops as an alternative food source from September through November, although there was no significant difference in August.