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The effects of light on sporangial production by Pilobolus crystallinus were studied using monochromatic light of three wavelengths and a wide variety of photon fluence rates and exposure durations. Plates of cultures grown for 9 d in darkness display ca. 1200 trophocysts, 40-90 very tall sporangiophores, and no sporangia. Exposure of these cultures to continuous blue light induced the formation of 50-90 short sporangiophores, followed by production of sporangia on both kinds of sporangiophores. Comparative wavelength effectiveness is partly a function of fluence rate and exposure time: At the two highest fluence rates (1.7 and 0.17 μmoles m² sec), 420 nm and 450 nm are equally stimulatory while 450 nm irradiation is the most effective light studied at the two lowest intensities (0.017 and 0.001 μmoles m² sec). Irradiation with 480 nm light was never observed to be the most stimulatory and usually was the least effective. These observations regarding wavelength effectiveness are similar to those reported for sporangiophore phototropism and in contrast to the wavelength dependence of trophocyst initiation in P. kleinii. Reciprocity was not observed; exposure duration appears to be far more important than intensity. Total fluences required for sporangial formation are approximately four orders of magnitude greater than those needed for phototropism.