Abstract: The euryhaline fish Oreochromis mossambicus was exposed to three sub-lethal concentrations of Cadmium ion, for 7 days to evaluate the role of protein catabolism in fulfilling the immediate energy needs of fishes under Cadmium ion induced stress. The levels of tissue protein, free amino acid, plasma ammonia and the activities of aspartate aminotransferase (AST), alanine aminotransferase (ALT) and glutamate dehydrogenase (GDH) were estimated in some of the vital tissues like gills, liver, kidney, muscle and blood of O. mossambicus. The rates of ammonia excretion, oxygen consumption and ammonia quotient (AQ) were also estimated. The significant (P< 0.05) decrease in the levels of proteins concomitant with remarkable increase in the level of free amino acids, ALT, AST and GDH activities in these vital tissues of fish species elucidated the protein catabolism as one of the main mechanism of meeting out the immediate energy demand of the fishes in condition of cadmium exposure. The AQ in treated fish increased significantly (P< 0.05), which indicate a marked increase in the catabolism of proteins during cadmium ions induced stress.