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Effects of ultraviolet (UV) irradiation on genetic inactivation and morphological structure of sperms were examined in the scallop, Patinopecten yessoensis . Haploid gynogenesis of the scallop was successfully induced by 50–60 s UV irradiation of 720 μW cm −2 s −1 . The fertilization rate apparently decreased with increasing irradiation time, and the development of the eggs fertilized with the genetically inactivated sperms terminated before reaching the D-shaped larvae stage. Scanning electron microscopy (SEM) showed clear destruction of the sperm acrosome and flagellum in the UV-irradiated sperms. As the duration of UV irradiation increased, the acrosome of sperms tended to suffer greater damage, until the sperms eventually lost their flagella. Abnormalities in these structures have appeared to account, at least in part, for the decline of the fertilization rate of eggs inseminated with UV-irradiated sperms.