Optical‐gain spectra from ZnO at 300 and 1.8 K have been recorded by a new modulation technique. The gain is extracted from amplified spontaneous emission by modulation of the excitation length and synchronous detection. The method is particularly suited for measuring gain coefficients in the range −100<g<500 cm−1. The observed gain spectra from ZnO are interpreted in terms of exciton‐exciton collision processes (300 K) and excitonic molecule transitions (1.8 K).