The results of experimental studies on the nonlinear evolution of perturbation waves in the turbulent wake behind a flat plate are presented. Sinuous perturbations at several amplitudes and frequencies were introduced into the wake by oscillating a small trailing-edge flap. The Strouhal numbers of the perturbations were specially chosen so that the downstream location of the neutral point (where the spatial amplification rate obtained from linear theory vanishes) was well within the range of measurements. The streamwise evolution of the waves and their effect on the growth of the turbulent wake was investigated. The amplitude of the coherent Reynolds stress varied significantly with x and changed sign downstream of the neutral point. This resulted in rather strong changes in the spreading rate of the mean flow with x. At high forcing levels, dramatic deviations from the square-root behaviour of the unforced wake occurred. Although the development of the mean flow depended strongly on the forcing level, there were some common features in the overall response, which are discussed. The measured coherent Reynolds stress changed sign in the neighbourhood of the neutral point as predicted by linear theory. The normalized mean velocity profiles changed shape as a result of nonlinear interactions but relaxed to a new self-similar shape far downstream from the neutral point. Detailed measurements of the turbulent and coherent Reynolds stresses are presented and the latter are compared to linear stability theory predictions.