Biotreatment of methylparathion (O,O-dimethyl-O-4-nitrophenyl phosphorothioate) was studied in aqueous mineral salts medium containing fungal culture to demonstrate the potential of the pure culture (monoculture) of Fusarium sp in degrading high concentration of methylparathion. A statistical Box–Behnken design of experiments was performed to evaluate the effects of individual operating variables and their interactions on the methylparathion removal with initial concentration of 1,000 mg/L as fixed input parameter. A full factorial Box–Behnken design of experiments was used to construct response surfaces with the removal, the extent of methylparathion biodegradation, removal of chemical oxygen demand and total organic carbon, and the specific growth rate as responses. The temperature (X 1), pH (X 2), reaction time (X 3) and agitation (X 4) were used as design variables. The result was shown that experimental data fitted with the polynomial model. Analysis of variance showed a high coefficient of determination value of 0.99. The maximum biodegradation of methylparathion in terms of the methylparathion removal (Y 1), chemical oxygen demand removal (Y 2) and total organic carbon removal (Y 3) were found to be 92, 79.2 and 57.2 % respectively. The maximum growth in terms of dry biomass (Y 4) was 150 mg/L. The maximum biodegradation corresponds to the combination of following factors of middle level of temperature (X 1 = 30 °C), pH (X 2 = 6.5), agitation (X 4 = 120 rpm) and the highest level of reaction time (X 3 = 144 h). The removal efficiency of methylparathion biodegradation was achieved 92 %. It was observed that optimum biotreatment of methylparathion can be successfully predicted by response surface methodology.