Two problems resulting from research on the subsistence of the Sakushu-Kotoni River site, a prehistoric 9th-century AD, early Ainu (Ezo) site in Hokkaido, Japan are examined. Almost 200,000 carbonized cultigen seeds were recovered by water flotation of soils. Most of the remains are seeds of barley, wheat, two millets, adzuki and mung beans, hemp, beefsteak plant, rice, and melon. Two implications of these data are examined. The first is that research on crop evolution in East Asia lacks systematic studies on archaeological cultigen remains. We report for the first time descriptive, taxonomic, and quantitative data on carbonized cultigen remains from northern Asia. In particular, the wheat is an unusually compact type. Second, the Ainu are usually portrayed as foragers who relied heavily on salmon and deer in historic times. The Sakushu-Kotoni River data demonstrate the existence of an agricultural phase early in Ainu history. We suggest that early Ainu village maintenance was facilitated by plant husbandry. The circumstances of the origins of Ainu agriculture are unresolved, and we question the complete disappearance of Ainu agriculture in the second millennium AD. -- AATA