A measurement of stimulated emission (SE) from ZnO thin films grown on c-plane sapphire by rf sputtering was performed. At 10 K, free exciton transitions were observed in the photoluminescence (PL), transmission and reflection spectra of the sample annealed at 950°C. SE resulting from both electron hole plasma formation and exciton-exciton scattering was observed at moderate excitation energy densities in the annealed samples. The observation of low threshold exciton-exciton scattering-induced SE showed that in rf-sputtered ZnO thin films, excitonic laser action could be obtained.