Abstract Microsporocytes of Lilium and Trillium can revert into a mitotic cycle when separated from the anther before the beginning of meiosis and cultured in vitro. Repeated short daily irradiations with visible blue light (10 6 ergs cm −2 sec −1) inhibit the onset of mitosis; the cells remain in interphase instead. The inhibition of cell division is reversed after the end of the light treatment. Meiosis is not affected by light. Part of the premeiotic G2 phase is light sensitive. No arrest of DNA-, RNA- or protein synthesis is involved in the photoinhibition of onset of mitosis. Irradiation with blue light decreases the α- absorption band of cytochrome a 3 but not of cytochrome a. This effect on cytochrome oxidase, however, is observed in all stages of meitotic prophase as well as in G2 cells.