In this paper the problem of perfect tracking of a reference trajectory generated by a neutrally stable exosystem is addressed for nonlinear systems. The main goal of the paper is to complete, with a feedforward unit, a standard feedback regulator designed to achieve the robust (with respect to a unknown parameter affecting the model of the system) asymptotic tracking of the reference trajectory. The feedforward unit is designed so that in case the unknown parameter assumes a known nominal value the perfect tracking is achieved, while, in the general case, the asymptotic tracking of the reference is guaranteed anyway. The analysis is carried out for minimum-phase nonlinear systems. Copyright (C) 1998 IFAC.