Diploid androgenetic Nile tilapia, Oreochromis niloticus, were produced using 5 min UV irradiation at 150 muW/cm and heat shock at 42.5degreesC for 3:30 min applied 25 min after fertilization. The survival rates to morula, pigmentation, hatching and yolk sac resorption stages were 42.84+/-4.73%, 10.69+/-9.24%, 2.03+/-0.60% and 0.07+/-0.07% (relative to controls), respectively. A significant female, but not male, effect on the survival was confirmed. Multilocus DNA fingerprinting produced by hypervariable 33.15 DNA probe verified the all-paternal inheritance in the androgens. The sex ratio of the androgenetic tilapia was not significantly different from the expected ratio of 1:0 (male:female), indicating that O. niloticus has a monofactorial sex-determining mechanism with female homogamety and male heterogamety. This study confirmed that the YY genotype is viable and. fertile in O. niloticus.