The photoperiodic response of the autumn onion cultivar ‘Bet Alpha’ and 4 other onion cultivars was studied. ‘Bet Alpha’ was the only one able to produce mature bulbs in the field under a daylength decreasing from 13 hours and 14 minutes to 10 hours and 40 minutes. The reversibility of bulbing and renewed vegetative growth under very short daylengths, and the requirements for relatively high light intensity and far-red irradiation for bulbing, were not basically different in Bet Alpha and in other cultivars. Under increasing daylength conditions ‘Bet Alpha’ was the earliest cultivar to mature. Very low critical daylength seems to be the essential characteristic of ‘Bet Alpha’, enabling it to produce mature bulbs under environmental conditions which do not permit completion of bulbing and maturation in other onion cultivars.