An internal model control (IMC) strategy for nonlinear single-input single-output systems is proposed. The controller is designed to provide nominal performance, and a nonlinear filter is added to make the controller implementable and to account for plant/model mismatch. An important advantage of the new approach is that the assumption of full-state feedback inherent in most input-output linearization schemes is eliminated. However, the proposed IMC strategy is restricted to open-loop stable systems with stable inverses. Under mild assumptions, the closed-loop system possesses the same stability, perfect control, and zero offset properties as linear IMC. Simulation results for a continuous fermentor illustrate the advantages of the nonlinear IMC strategy.