Activity levels of acid phosphatase (ACPase) in midgut gland and muscle tissues of penaeid prawn, Metapenaeus monoceros were studied after exposure to selected sublethal concentrations of phosphamidon after acute and chronic exposure. Phosphamidon was found to elevate the activity levels of ACPase in the selected tissues. The rate of elevation of ACPase is dose-dependant and is more at higher concentrations. Acute and chronic exposure to phosphamidon creates disturbances in the normal functioning at the cellular level in the tissues of prawn, M. monoceros.