This paper investigates problems associated with multimode oscillations in semiconductor lasers. In particular, even in semiconductors there can exist a spatially nonuniform distribution of excess electrons and holes because a given mode de‐excites electrons most strongly at antinodes and not at all at nodes of the electric vector. This nonuniform distribution encourages the oscillation of other modes which have a different distribution of the electric field vector and which, in particular, have finite electric fields at nodes of the first oscillating mode. Diffusion of electrons and holes tends to wipe out nonuniform carrier distributions. For low temperatures, this diffusion is relatively slow, and for current densities only moderately above the threshold, several modes may oscillate simultaneously. At higher temperatures the suppression of additional modes is much more effective. Because of the complexity of the calculations, the present work had to be restricted to Fabry‐Perot type modes and relatively low power levels. Comparisons are made between theory and experiments.