Freshwater fishTilapia mossambica (Peters) were exposed to sublethal alkaline water (pH 9·0) and the branchial protein metabolism was studied on acute exposure and acclimation. Branchial tissue had elevated structural and soluble protein fractions on acclimation which was correlated towards the gill hypertrophy. Proteolytic activity of the tissue was elevated on both acute exposure and acclimation. A/O ratio which forms a measure of ammonia formed per unit O2 consumption was lesser on acclimation and hence mobilization of tissue free ammonia towards glutamine formation was suggested. The metabolic modulations have been correlated towards the positive survival value of the fish in alkaline waters.