Fertilization with UV irradiated spermatozoa induced inviable gynogenetic haploid development in ocellated puffer Takifugu rubripes. Cold treatment (0°C, 45 min duration) initiated 5 and 30 min after fertilization with UV irradiated spermatozoa gave viable gynogenetic diploid progeny. Heat treatment (30°C, 5 min duration) from 5 min after fertilization also produced viable diploid gynogens. Since the resultant fish showed a heterozygous genotype which occurred by gene-centromere recombination as well as two homozygous genotypes at aconitate hydratase or isocitrate dehydrogenase isozyme coding locus, they were considered diploid gynogens which were produced by inhibition of the second polar body extrusion. Control progeny exhibited 1 : 1 sex ratio between females and males, while 93% of the gynogens were female. This result suggests that this species has basically XX-XY sex determination system.