In 1968–1970 Pleurotus ostreatus sporophores produced most spores in the afternoon during late fall and winter, when production increased with daily increases in temperature and decreases in relative humidity. In late summer and fall, production peaked between late afternoon and midnight. Diurnal production patterns did not occur in the laboratory at constant temperature, light, and relative humidity. At 85–95% relative humidity, sporulation increased exponentially with temperatures from 1 to 25C. Production ceased at less than 1C and above 27–30C. Alternating temperatures of 5 and 25C at 12-h intervals resulted in corresponding periods of high and low spore production which continued for up to 24 h after treatment ceased. At 17C, sporulation was higher at 30% relative humidity than at 90% relative humidity. Sporulation was unaffected by alternating light and darkness at constant temperature and relative humidity.