The bamboo bug Notobitus meleagris F. shows two types of mating aggregation: harem including a male and one or more females, and multi-male including some males and females. Both types of mating aggregations were formed only on young bamboo shoots. The mean body size of males was significantly smaller in multi-male aggregations than in harem aggregations, but female size was independent of aggregation type. There was a significant positive correlation between the body size of males and the number of females in harem aggregations that formed in association with male-male aggressive contests. More females were attracted to thicker than thinner bamboo shoots, where larger aggregations formed. Multi-male aggregations were observed more on thicker than thinner shoots. Multi-male mating aggregations may occur when females cannot be economically defended.