Temperature-induced sex reversal has been widely reported in Nile tilapia. While most studies under controlled conditions show that high temperatures can masculinize XX individuals, rare studies suggest a feminizing effect of such treatments in YY genotypes. The effects of low temperature treatments have rarely been explored. In the wild, this species can be exposed to various thermal regimes/ranges that may influence sexual differentiation of undifferentiated fry. Temperature monitoring over a year in the shallow areas where undifferentiated Nile tilapia fry live revealed periods of high (32-42°C) and low (12-20°C) temperatures in Lake Kou. Simulation of "hot periods" (27-36°C) under controlled conditions induces a high proportion of males compared to the control group (27°C). Sexual genotyping using amh markers, specific for X and Y chromosomes, confirmed the masculinizing effects of thermal fluctuations and allowed the identification of an XX male in Lake Kou. Progeny tests confirmed this sexual genotype. These results suggest the probable masculinization of thermosensitive XX fry induced by high temperature fluctuations in the shallow areas of the lake. Regarding cold temperatures, although constant treatment at 20°C leads to a balanced sex ratio, some XY females were nevertheless identified in the treated batch. However, no XY females were identified among the wild-caugth individuals in the lake, questioning the physiological reality of a possible feminizing effect of low temperatures.