In many animal species, including insects, the morphology of the male genitalia is diverse. One of the most enigmatic observations in evolutionary biology is the evolution of morphological traits of male to physically injure female. It is unclear why males inflict the wound, but it has been suggested that males evolve harmful traits to induce favorable responses for males, such as delayed remating or increased rates of oviposition. The male West Indian sweetpotato weevil, Euscepes postfasciatus (Fairmaire) (Coleoptera: Curculionidae), possesses spines on the endophalus of the intromittent organ, and this spiny area is able to divide into two types. One is a band-shaped area with many short spines, and the other is an island-shaped area with several long spines. Both the standardized shape of the spines or the number of spine are significantly different between the two areas. These traits of male genital spines would affect the male reproductive success through the response of the partner. To explain the role of harmful male genital traits, we investigate the relation between the morphological character of male genitalia and fitness in E. postfasciatus. We will discuss the adaptive significance of the harmful genitalia in this weevil.