In this study, a Hertwig effect with a non-typical biphasic curve was obtained using sperm irradiated with increasing intensities of UV. The first phase of the UV curve appeared to be quite different from that normally demonstrated using γ or x-ray irradiation. This difference is characterised throughout the length of the first phase by (1) low and stable embryo hatching rates of about 3.5%, and (2) exclusive formation of haploid embryos at any irradiation intensity. Additionally, at both phases, the ability of the sperm to induce morula formation was not affected at all, and no aneuploidy nor chromosomal fragments could be seen. Therefore, it was suggested that in this fish the lethal effect of UV irradition on sperm is mainly expressed on early differentiative events during embryogenesis, which lead to a degeneration of the embryos during early stages of their development. The possible mechanism by which haploidy is achieved during the first phase is discussed. Two generations of diploid gynogenetic tilapias were induced by activating Oreochromis aureus eggs with UV-irradiated O. niloticus sperm and by using the heat-shock technique, at optimized conditions, for the prevention of the second polar body extrusion. Species specific dominant genetic markers (serum esterases and tail striation) were used to confirm the exclusive content of the maternal genome in gynogenetic offspring. Very low survival rates (0.36%) were shown in F1 gynogenetic fish as well as a high incidence of malformations among survivors. In the second gynogenetic generation, both significantly higher survival rates (3.6%) and a considerably reduced incidence of malformations were obtained. We suggest that low frequencies of recombination occur in this species and cause a rapid increase in the inbreeding level. This is followed by the expression of lethal and defective genes that are considerably reduced after second generation selection.