Male dimorphism has been thought to correlate with alternative reproductive behaviors. Alternative reproductive behaviors promote asymmetry in sperm competition, and the differences in fertilization success could promote adaptations in ejaculate characteristics in relation to each reproductive behavior. Here, using allometric analysis, we show that there is clear ejaculate dimorphism in males of the squid Loligo bleekeri, a species with body-size related alternative mating behaviors. A morphological switch point was detected in internal characters: larger individuals made discontinuously longer spermatophores than smaller individuals, although any switch point was not detected in external characters (fin length, fin width, head width, mantle width, tentacle length and hectocotylus length) except bimodal body size. This clear internal switch point could be an adaptation to the characteristic alternative mating behaviors of loliginid squid, in which males use different mating tactics to pass spermatophores to different sperm storage sites in and on the females. This study reports on clear dimorphism in ejaculate characteristics in cephalopods for the first time, to our knowledge. Our results indicated that alternative reproductive behaviors can result in morphological adjustment in internal characters.