Abstract: Sub lethal toxicity of phenolic compounds in a tropical teleost fish Oreochromis. mossambicus was investigated. Experimental animals were exposed to sub lethal (1/10th of 96 hr LC50) concentration of phenol (3.12 mg l-1) and m-cresol (2.2 mg l-1) for a period of 21 days using a semi-static test bioassay system. Significant (P<0.05) lack of cortisol response was observed on exposure to both the phenolics. Furthermore total carbohydrate significantly (P<0.05) decreased in liver and muscle and blood glucose level was also significantly (P<0.05) decreased. The LDH (lactate dehydrogenase) showed significantly (P<0.05) elevated activity in the liver and kidney of fishes treated with phenol suggesting anaerobic glycolysis. Elevation in the activity (P<0.05) of ALT (alanine amino transferase) can be considered as a response to meet the excess energy demand. Increased serum ACP (acid phosphatase) activity suggests an increase in lysosomal mobilization. Also a significant (P<0.05) inhibition of branchial ATPases (Na+K+-ATPase, Mg2+-ATPase and Ca2+-ATPase) was observed. Moreover gill histopathological analysis on phenol exposure revealed severe lesions such as architectural loss, necrosis, desquamation of epithelial layer, hyperplasia and telangiectesis. Significant changes observed on m-cresol exposure were lamellar necrosis, lamellar shortening, telangiectesis and lamellar clubbing. Phenolic compounds even at very low concentrations (1/10th of 96 hr LC50) for a short duration of 21 days induce metabolic stress, impair branchial functioning and are likely to induce tissue damage in O. mossambicus.