Theoretical models predict that the sex allocation of hermaphrodites (i.e., the proportion of reproductive resources allocated to the male function instead of the female function) is affected by the number of individuals that can donate sperm to each other (mating group size, MGS). Moreover, when dwarf males coexist with hermaphrodites in a population (androdioecy), dwarf males are predicted to appear only when MGS is small. However, little empirical study has tested these predictions simultaneously. We studied the androdioecious barnacle Octolasmis unguisiformis, in which hermaphrodites carry dwarf males, to test these predictions. The male (the total volume of the testis and seminal vesicle) and female (the ovary volume) functions of mature hermaphrodites responded positively and negatively to MGS, respectively. Thus, the sex allocation of hermaphrodites is correlated positively with MGS. In addition, dwarf males occurred when MGS was small. These results support the theoretical predictions and, therefore, indicate that the barnacle individuals show adaptive sex allocation.