We report the measurement of third-order optical nonlinearity (χ ) of ZnO microcrystallite thin films near the excitonic resonance at various temperatures using the femtosecond degenerate four-wave-mixing technique. The measured φ response times are 270, 240, and 160 fs at 4.2 K, 77 K, and room temperature, respectively. The values of φ range from 10 to 10 esu. The maximum values of χ always correspond to the absorption peaks at different temperatures. Room-temperature excitonic enhancement of χ is also observed. © 1999 American Institute of Physics.