We propose a rate equations model for the description of a mul tilongitudinal mode semi- conductor laser. We show how usually disregarded effects su ch as parametric interaction, carrier-diffusion and interference between the fields of thedifferent longitudinal modes play an important role in the final operating characteristic s of the laser. Parametric inter- action induces a non-trivial distribution of spectral poweramong different cavity modes. Interference results in nonlinear gain terms in the equatio ns, which favour longer wave- lengths versus shorter ones. Carrier-diffusion is regulati ng the magnitude of such effects. In the limit of very strong diffusion, previous rate equatio ns models are recovered.