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Seed production of Fagus crenata was visually monitored at 159–321 districts in the Tohoku Regional Forest Office, which covers almost all the natural forest of this species in north-eastern Japan (ca. 200km×400km), for 12 years from 1989 to 2000. Flowering and seed production of F. crenata were also investigated at three stands using seed traps during the same period, to calibrate visually assessed data with actual seed production, and to estimate factors affecting seed production of this species in more detail. Geographical distributions of seeding in this region were classified into four types: mast seeding throughout the study area, in northern districts, in southern districts, and in no districts, and these types occurred in alternating years. Mast seeding throughout the study area occurred only twice. These two events might possibly be explained by higher temperature in mid-June of the preceding years. There were …