The sexual differentiation of P. melanogaster embryos was examined among fish kept in glass aquaria (80x40x35 cm) at 23, 26and29°CwithapH=6.5.Analysis of variance of the data revealed a significant variation (at the 5 % level) of sex composition relative to increase of water temperature .Two mathematical representations were predicted for males and females.Accordingly, the degrees at which all males, or all females, or 1:1 sex ratio had been determined was 18.95, 32.01 and25.4°C respectively. The mechanism of this temperature effect on the sex ratio was discussed where growth offish was suspected to have played a role in this process.