An understanding of the mode of pollination in basal angiosperms can provide insights into the early development of insect-pollination in angiosperms. Acorus (Acoraceae) is the earliest diverging clade of the extant monocots. The pollination of A. calamus has long been enigmatic because the plant’s insect-pollinators have never been observed, and its fruit production is rare in the wild. We, therefore, investigated the flower visitors and fruit set of A. calamus in a Japanese population. Pollinator observations and insect-exclusion experiments showed that A. calamus was pollinated mainly by insects and on rare occasions, by wind. Two kateretid beetle species were frequently observed on A. calamus inflorescences; the adults mated and laid eggs, and the hatched larvae fed on pollen and other plant tissues on the inflorescences. Consequently, the adult beetles carried a large amount of pollen on their bodies. Therefore, kateretid beetles could act as important pollinators for A. calamus, although further studies are required to clarify the roles of brood-site pollination mutualism.