The lysosomal system in bivalves is sensitive to changes in the intra and extra cellular environment. The activity pattern of hemolymph acid phosphatase, a marker enzyme of the lysosomal system, was studied in the green mussel (Perna viridis) exposed to sub lethal concentration of 25 μg/l of copper and mercury at 24 hr, 72 hr, 7th day and 14th day. The results of the study revealed that at the sub lethal level, copper can induce remarkably higher acid phosphatase activity (hypersynthesis) than mercury. Hypersynthesis of the enzyme occurs much earlier in copper treated mussels than observed in mercury exposed mussels, but on continued exposure, the acid phosphatase activity shows a considerable decline.