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We present experimental results on the spectral linewidth of one longitudinal laser mode of multimode semiconductor lasers with different emission wavelengths and different wave guiding. The dependence of linewidth on mode power cannot be described by a single-mode theory (modified Schawlow-Townes formula), for all lasers. The observed power independent contribution to the linewidth as well as the slope of the linewidth versus optical power data show significant multimode effects. A theoretical model for the linewidth of a two-mode laser provides a qualitative description of the experimental results.