The paper deals with the fuzzy control of a wastewater treatment process in which the organic substances are removed. The process is treated as a multivariable process having as control inputs the dilution rate, the aeration rate and the recycling rate and as output variables the dissolved oxygen and the substrate concentrations in the effluent. The process sensitivities with respect to the control inputs in the case of indirect control through the control of dissolved oxygen concentration and of the direct control of substrate were studied. In all cases fuzzy controllers were used. It was also shown that fuzzy control provides good results in the presence of disturbances and parametric uncertainties of the model.