In this study, the effect of temperature on survival and sex ratios of guppy Poecilia reticulata (Peters 1860) was investigated. Treatments of 19, 21 and 22.5°C low and 29, 32 and 35°C high water temperature were used on fry and gravid females for 11 days after parturitions and several days starting from the 16th day after first parturition until second parturition respectively. The high water temperature treatments caused the death of gravid females. Survival rates of heat‐treated fry were generally decreased with increasing and decreasing temperature. The sex ratios of the progenies from survived gravid females were not differed from a balanced sex ratio (P>0.05). The proportion of females increased gradually with decreasing temperature and the proportion of males increased gradually with increasing temperatures rather than having a threshold response in heat‐treated fry. However, mortality of heat‐treated fry was high; it is most likely that our results were biased by sex‐dependent mortality, which were also revealed by corrected sex ratios of the treatment groups and progeny testing of some individuals. These results suggest that a major gene linked to X‐chromosome could be responsible for resistance and sensitivity to both high and low temperature.