SUMMARYFormation of basidiocarp initials inPsilocybe cubensisoccurred only when cultures were illuminated. Short durations of light (0.0025 sec of xenon-arc flash) were sufficient for initiation. Light-induced initiation was saturated at a dose of 0.345×104ergs/cm2at 460 nm. UV and blue wavelengths of 370, 440, and 460 nm were the most effective. Green and red wavelengths greater than 510 nm were ineffective.