The observation of large nonlinearity of electronic origin and the measurement of the very fast response time of this nonlinearity are reported. The magnitude and dynamics of this excitonic effect were measured at various temperatures in high-purity ZnSe thin film grown by molecular beam epitaxy on GaAs substrates. Two series of experiments were performed. In the first, a 3-ns laser system was used to measure the nonlinear index of refraction as a function of wavelength at 150 and 300 K. Clear excitonic effects were observed. The experimental results agree well with a partly phenomenological plasma theory that takes into account many-body effects. In the second series of experiments, the response time of the excitonic nonlinearity was measured using femtosecond laser techniques. A very rapid response time for the nonlinearity (exciton recovery time) in the range of 100 ps was measured. The exciton recovery time was faster for thinner samples.