The interaction of two light waves, having different frequencies and propagating in a dispersive nonlinear medium, is studied using the method of Zakharov (Zh. Eksp. Teor. Fiz. 51, 1107 (1966) [Sov. Phys. JETP 24, 740 (1967)]). This method does not require the sidebands of the incident waves to have frequencies comparable to those of the incident waves, as do the coupled nonlinear Schrodinger equations that are normally used to model this interaction. It is shown that cross-phase modulation does not necessarily lead to instability of the incident waves. In particular, two light waves propagating in the normal-dispersion regime of a conventional single-mode fiber are stable