Relatively few flowering plants show ambophily (pollination by both wind and insects), and whether and when ambophily is advantageous has not been studied well. In the present study, we report ambophily in two dioecious pioneer tree species, Mallotus japonicus Müll.Arg. in a temperate forest of Japan, and Mallotus wrayi King ex Hook.f. in a tropical forest of Borneo, and discuss the conditions that contribute to the maintenance of ambophily. Both species are pollinated by wind because they set fruits even when flower visitors were excluded and because substantial amounts of airborne pollen reached female trees. Insects may also contribute to fruit set, because insects with body pollen visited female inflorescences. Because M. japonicus and M. wrayi exhibit floral characteristics that are adapted to both wind and insect pollination, ambophily may be actively maintained in the two species at the study sites and perhaps elsewhere. Whereas previous studies have indicated that ambophily is advantageous for pioneer plants because of changing wind conditions during forest succession, our preliminary data suggest that changes in population density also contribute to the maintenance of ambophily in M. japonicus.