In species of several insect orders and spiders, it has been shown that the size of male genitalia relative to body size decreases as the body becomes larger (negative allometry), while the relative size of other morphological traits tends to be constant. Such a contrast between genital and somatic traits suggests stabilizing sexual selection on male genitalia: males with small or large genitalia are prone to fail to inseminate females due to incompatibility of their genitalia. In the present study, we tested the contrast between genital and somatic traits for males of a lepidopteran insect, Ostrinia latipennis. We examined allometry of five genital and 11 somatic traits for each of three local populations of O. latipennis. Of the 15 allometric slopes for genital traits, 14 showed significantly negative allometry, whereas none of the 33 slopes for somatic traits represented negative allometry. These results showed that the size of male genitalia in O. latipennis is more stable than the size of somatic traits against changes in body size. This study supports Eberhard et al.'s (1998) hypothesis which states that the low genital allometry in insects and spiders is caused by sexual selection. Based on currently available information on genital morphology and sexual communication in O. latipennis, it seems unlikely that the lock-and-key hypothesis is responsible for the stable genital size in this species.