[en] The nonlinear optical response of excitons in a single ZnSe quantum well is investigated by pump-probe measurements. In the counter-circular polarization configuration at negative delay, Δt, spectral oscillations are observed the period E_PP of which is energy dependent. Whereas at energies above the 1S exciton the usual E_PP=h/|Δt| dependence is found, the period strongly deviates from that value at energies below the exciton. Model calculations reveal that this behavior is caused by the bound biexciton state in the four-particle correlations. (c) 2000 The American Physical Society