In Oreochromis niloticus, hormonal sex reversal of an entire progeny from crosses between a sex reversed XX male (produced by steroid treatment) and a normal XX female, resulted in a progeny of sex reversed XX male siblings. The sex ratio of the progenies produced by single pair matings of 22 of these sex reversed males is only rarely all-female, as opposed to predictions of a monofactorial sex determination model. In order to assess a potential thermosensitive differentiation, progenies from these sex reversed males were submitted to high temperatures for 21 days. Nine to 13 day-old fry post-fertiIization (PF) from 17 progenies were reared at high temperatures ranging from 30 to 36°C and to an average control temperature of 28°C, respectively. No significant difference in survival was observed between the two groups of fry with 76.2 and 74.2% survival rates at 28 and 30-36°C, respectively. All surviving fry (an average of about 100 per batch) were sexed by histological examination of tissue squashes at 60-90 dPF. High temperatures shifted the control sex ratios from 0 to 91%. As the low mortalities cannot account for these deviations, thermosensitivity is demonstrated in O. niloticus.