AbstractFruit and seed set in a dioecious tree,Phellodendron amurensevar.sachalinensegrown in isolation, were studied for three years in Hokkaido, northern Japan. Hand pollination did not affect fruit set during the three years, but did affect seed‐set by increasing the proportion of fruits with higher seed numbers. Artificial defoliation resulted in decreased mean seed mass, but caused no significant differences in mean seed number per fruit. Total seed mass per infructescence increased with inflorescence size except with artificial defoliation where it reached an upper limit. Although the results of the artificial defoliation treatment indicate that resources are most important for fruit and seed set, it is concluded that fruit and seed set of the tree are not limited by resources, since the number of fruits increased with inflorescence size and an artificial defoliation treatment did not decrease seed‐num