We present the temporal and spectral evolution of stimulated emission from (Zn,Cd)Se/Zn(S,Se)/(Zn,Mg)(S,Se) multiple quantum well lasers at 2 K after picosecond optical excitation. For pumping well above the threshold, ultrahigh-frequency oscillations of up to 100 GHz are observed. These oscillations are generated by a multimode emission through mode beating caused by the coherent superposition of longitudinal ground modes and higher lateral modes. The modelling of the experimental results shows that the modes are coupled to some extent. ©1997 American Institute of Physics.