We examined melon-headed whales that mass-stranded live in two events in Japan: (1) 171 animals at Tanegashima Island in 2001 and (2) 85 animals at Hasaki in 2002. We report here the results of life history traits and group composition of these strandings, and compare them to another mass stranding with 135 individuals at Aoshima in 1982. In the Hasaki event, most stranded animals, including those released were sexed and measured. The proportion of live males released was much higher than that of females, and larger animals, especially females, were more likely to have died. Females were estimated to attain sexual maturity at around 7 yr and give birth every 3–4 yr. The sex ratio was significantly different between the Hasaki and Aoshima events. Among dead specimens, females of various age classes were included in all strandings, while age distribution of males varied considerably among strandings. This suggests females show group fidelity while males move between groups. Asymptotic body length of females from Hasaki was significantly smaller than that from Tanegashima, suggesting that more than one population of melon-headed whales exist off Japan.