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Summary form only given. Resonant cold atomic samples are known to provide an efficient medium for the study of nonlinear optical processes such as four-wave-mixing (FWM) and phase conjugation (PC). Although this system is frequently modeled as an homogeneous sample of two-level atoms, the actual nonlinear response strongly depends on the internal structure (Zeeman degeneracy) of the atomic energy levels and consequently on the polarizations of the optical fields. We have studied the spectra of quasi-degenerate PC on the 6S/sub 1/2/(F=4)-6P/sub 3/2/(F=5) transition of cold Cs atoms for different optical polarizations. The atomic sample was obtained in a magneto-optical trap. The spectra where recorded within a 1 ms time interval during which the trapping light beams were blocked and the quadrupole magnetic field was turned off. We used a backwards FWM scheme with two counter-propagating pump beams and a probe beam. The PC signal propagates in opposite direction respect to the probe beam. Typical PC spectra as a function of frequency offset are shown for probe beam polarization parallel and perpendicular to pump polarization. Large differences are observed between these two cases demonstrating the crucial role of the Zeeman degeneracy on the atomic response. The experimental spectra are in good agreement with the theoretical predictions for FWM generation in degenerate two-level atoms.