Phenoloxidase (PO) activity, cellular indices and total plasma protein of haemolymph of black tiger shrimp, Penaeus monodon (6.2±0.84 g) exposed to three different sublethal concentrations of a crude oil were investigated. The experiment was conducted in a flow-through oil contaminated seawater system and the concentrations of the toxicant were assayed based on the means of naphthalenes in the crude oil at 0.455, 0.240 and 0.129ppm. The experimental animals were tested at 10different intervals (0, 1, 3, 6, 12, 24, 48, 96, 192, and 384 h) after exposure. In this study the element of recognition (phenoloxidase activity) and effector branches (haemocytes) of immunesystem of black tiger shrimp were significantly (P<0.05) decreased during exposure to sublethal concentrations of crude oil. Results of the present study on the changes in total haemocyte counts, differential cell counts, and also PO activity reveal that sublethal concentrations of crude oil had a suppressive effect on important immune constituents of black tiger shrimp which suggests predispose the shrimp to immune-mediated diseases.