The mating behavior of captive Loligo bleekeri and the paternity of the resulting progeny were examined based on behavioral observations and genetic analyses. In this species, there are three mating behaviors (male-parallel, head-to-head, and extra-pair), and two sperm storage sites in females (seminal receptacle and the opening of the oviduct), which suggest that sperm competition occurs. All three mating behaviors were observed, and females mated often with different males, resulting in multiple paternity within 3 of the 4 broods examined. In each brood, the male to mate last and frequently before the female spawned fertilized the most eggs (87-100%). A sneaker male that mated by extra pair copulation sired 8.5% of the eggs in a brood. Some eggs were fertilized by sperm received before the start of the study, indicating that sperm can be stored for at least several days before a spawning. In the broods with multiple paternity, the paternity patterns differed among egg capsules. Male competition was more intense between similar-sized males than between different-sized males, but body size did not affect the copulative success in the male-parallel position. We found the multiple mating and multiple paternity in L. bleekeri, and the paternity rates differed depending on the complex factor with mating position, timing, frequency and duration, relating with alternative reproductive behaviors.