A characteristic effect of stimulated emission of radiation1 in a fluorescing material i s the narrowing of the emission line as the excitation is increased. We have observed such narrowing of an emission line from a forward-biased GaAs p-n junction. As the injection current is increased, the emission line at 77°K narrows by a factor of more than 20 to a width of less than kT/5. We believe that this narrowing is direct evidence for the occurrence of stimulated emission...