Lepidopteran males produce two types of sperm, nucleated eupyrene sperm and non-nucleated apyrene sperm. These sperms were transferred to the bursa copulatrix of the female within a single spermatophore at mating. Therefore, both types of sperm migrate to the spermatheca. As the adaptive significance of apyrene sperm migration to the spermatheca, the cheap filler hypothesis that highly motile apyrene sperm in the spermatheca reduce female receptivity to re-mating has been developed in studies of polyandrous species. However, males of monandrous species also produce apyrene sperm. Thus, study on apyrene spermof monandrous species would be valuable to clarify the role of apyrene sperm. The sperm dynamics in females and the motility of eupyrene and apyrene sperm in the spermatheca were examined in the monandrous swallowtail butterfly, Byasa alcinous, in Japan. Sperm migration was occurred for eupyrene and apyrene sperm within 1 day after copulation. More than 10,000 apyrene sperm and 5,000 eupyrene sperm remained in the spermatheca at least 8 days after copulation. While eupyrene sperm maintained their motility for 8 days, apyrene sperm lost their motility soon after the arriving at spermatheca, and no motile apyrene sperm were found 8 days after the copulation. Since the cheap filler hypothesis requires the apyrene sperm in the spermatheca to show high motility, an alternative hypothesis that apyrene sperm assist migration of eupyrene sperm from spermatophore to spermatheca or that apyrene sperm ingested as nutrition by female for monandrous species was discussed.