Nuclear changes in normal and gynogenetic eggs of the Chlamys farreri were examined under a fluorescence microscope during meiosis and fertilization. Two milliliters of sperm suspension ( 1 ?107 sperm? mL -1) were spread on a 9. 0 cm diameter plastic petri dish and placed on a reciprocal shaker 15 cm below a 15 W UV germicidal light. Haploid gynogenesis was induced by sperms , which were ultraviolet (UV) 2irradiated for 30 s at an intensity of 2561μW?cm -2 ?s -1. Both the treated sperm and the untreated sperm (control) were mixed separately with a suspension of eggs ( 2 ?104 egg?mL -1) , and the eggs were then allowed to develop at 22 ℃. Samples of eggs from the two groups were taken at 5 min intervals up to 2 h after the addition of sperm and fixed in 2 % formalin in seawater at 4 ℃. After rinsing three times with 0. 1 mol?L -1 phosphate buffer containing 8 % sucrose , the samples were stained with DAPI , and observed with a fluorescence microscope. The results obtained in this study demonstrated that although UV irradiation did not affect either meiotic maturation or the formation of the male and female pronuclei , their developmental progress was delayed apparently. At metophase of the first cleavage , the male pronucleus derived from sperm genome inactivated by UV -irradiated sperms inseminated with the normal egg formed no chromosomes , unlike the female pronucleus , but became a dense chromatin body (DCB ) , which did not participate in the karyokinesis at anaphase of the first cleavage as evidenced by chromosomal nondisjunction. On completion of cytokinesis of the first cleavage , the DCB was seen either in the cytoplasm of one of the two blastomeres or on the equatorial plate as two partitional parts. As did the normal eggs , the eggs inseminated with UV -irradiated sperm also developed in an asynchronous manner. On the other hand , in this study the eggs inseminated with UV -irradiated sperm developed slowly , and time difference between normal and gynogenetically developed eggs in reaching a developmental progress. Cytological evidence of the induction of gynogenesis in the scallop was first demonstrated.