Abstract.— The effects of thermal and thermo/haline shocks on the tilapias Oreochromis mossambicus and O. aureus × O. niloricus hybrids were investigated. Fingerlings were transferred abruptly from 25 C fresh water to fresh water or salt water (26 ppt) maintaining temperatures of 15, 20, 30 and 35 C. In fresh water, fish survival was not affected by temperature shock, but they suffered from a chill coma and developed fungal infection at 15 C. In salt water, thermo-haline shock did not affect the survival of O. mossambicus, while the survival rates of tilapia hybrids were 6.7, 100, 100, 70 and 59.7% at 15, 20, 25, 30 and 35 C, respectively. Plasma osmotic concentrations of O. mossambicus transferred to 15 and 35 C salt water were quickly and sharply increased, but gradually decreased to lower levels. Osmotic concentrations of tilapia hybrids were also increased, but never returned to their initial (lower) levels over one week. This study revealed that these fishes are more tolerant to high rather than low temperature shocks and O. mossambicus are more resistant to thermo-haline shocks rather than tilapia hybrids. It also indicated that thermo-haline shocks should be minimized during fish transportation and acclimation.