Unstratified seeds of Betula pubescens Ehr. and B. verrucosa Ehr. exhibited low or no germination at temperatures from 10° to 20 °C. unless exposed to continuous illumination or illumination interrupted only by a short daily dark period. Germination was high even under a short primary light period if the long dark period was divided into two short ones by an additional daily light period. Besides these true photoperiodic responses, germination was slightly correlated to illumination intensity and to the amount of light. Germination was somewhat reduced when the illumination intensity was either decreased or increased from an optimum. At temperatures of 20° to 25 °C., germination was fair, even under a short daily light period. At still higher temperatures no light was needed for maximum or nearly maximum germination. Seeds of three other tree species also germinated better if exposed to long light periods, but the differences were small or not significant statistically. In most of the experiments the effect of temperature was separated from that of light by using running water as the germination medium. The antibiotic Rimocidin as well as various other chemicals and ultrasonic treatments failed to affect the germination of B. verrucosa.