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The objective of this study was to evaluate the effect of chlorpyrifos, an organophosphate pesticide, on immune response parameters of Nile tilapia (Oreochromis niloticus). The results indicated that chlorpyrifos (0.051, 0.102 and 0.255 mg L-1) did not provoke significant changes in the proliferation capacity of lymphocytes of tilapia; however, at 0.051 mg L-1, this pesticide induced a diminishment in concentration of lgM in plasma. On the other hand, organisms exposed to high concentration of the pesticide showed an increase in the lysozyme activity compared to control fish.