Optimal sperm allocation should differ according to the level of polyandry within a population, because the risk of sperm competition depends on the re-mating frequency of females. We compared the number of sperm ejaculated by males into the female reproductive organ between strains with different levels of polyandry in the adzuki bean beetle, Callosobruchus chinensis (Coleoptera: Bruchidae) when males were reared in different larval densities in a bean. The results showed that males derived from a population with a higher level of polyandry increased ejaculatory expenditure when they were reared under higher larval densities. We discuss the evolutionary correlation of ejaculatory expenditure to the level of polyandry.