Gynogenetic diploids were induced in flounder, Paralichthys olivaceus, by applying cold shocks to the eggs after insemination with sperms which genetically inactivated by ultraviolet rays. Sperms were obtained from black sea bream, and also used flounder and mud loach as controls. Embryos inseminated with sperm, irradiated with dose varing from 3000 to 4500 erg/mm2, showed the highest incidence of haploid syndromes. Fertilized eggs were treated with cold shock(0~4。C) for 45min for blocking the second polar body. Optimum temperature for the highest incidence of gynogenetic diploid was revealed at 2。C. in the cytogenetic studies, there was no difference of karyotype or number of chromosome between the induced gynogenetic diploids and normal diploid controls. All gynogenetic individuals had 48 acrocentric chromosomes and nucleus volume of erythrocyte was 13.36μm3, Gynogenetic diploids showed slightly higher rates of survival and growth than the normal diploids for 7 month rearing period. The gonad of seven-month-old gynogenetic diploids had the normal external morphology and active meiosis occurred normally in their ovaries.