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Temperature selection of two fresh water fishes Aphanius dispar (Ruppel 1828) and Gambusia affinis Holbrooki (Girard 1859) was investigated in a thermal gradient. One animal at a time was studied in three temperature gradients extending from 6 to 40-degrees-C (Low range 6-21-degrees-C; Medium range 13-31-degrees-C; High range 21-40-degrees-C). Most of A. dispar selected temperatures between 14-15-degrees-C in the low range, 19-22-degrees-C in the medium range and 26-32-degrees-C in the high range, while G. affinis selected temperatures between 13-20-degrees-C in the low range; 22-30-degrees-C in the medium range and 22-32-degrees-C in the high range. The mean critical thermal maximum (C.T. Max.) for A. dispar and G. affinis was 38.1-degrees-C and 38-degrees-C respectively. The mean critical thermal minimum (C.T. Min.) was 4.2-degrees-C and 4.4-degrees-C for A. dispar and G. affinis. Both species showed a tolerance to high temperature, but the introduced species, G. affinis had a wider range and is better suited to adopt to the desert habitat, and hence could constitute a threat to the endemic species A. dispar.