The investigation concerns the effects of five factors upon the tendency of onion sets to bolt and the production and ripening of bulbs. The factors studied fall into three categories: (1) conditions under which seedlings were grown to produce sets, (2) conditions of storage of sets during dormancy, (3) conditions of growth during the second season. The first comprised sowing date of seed (day-length and temperature effects) and temperature of growth while forming sets; the second, high-and low-temperature storage for different periods and throughout the winter; the third, temperature and day length during the second season. Field trials and experiments with controlled environment were carried out. The main results of the former were as follows: Large sets were much more liable to bolting than small. Sets produced at high temperature under glass were practically free from bolting when grown on, irrespective of the storage conditions. Storage throughout the winter (22 weeks) at 30d̀ C. practically eliminated bolting; 30d̀ C. for the first 8 weeks reduced bolting but given for the last 8 weeks was much less effective. Cold storage (od̀ C.) throughout or for the first 8 weeks considerably reduced bolting, but when given for the last 8 weeks tended towards increase. High-temperature storage leads to loss in weight; the maximum loss in any treatment, 75%, was associated with 77% mortality in the field. This detrimental effect on survival is confined to sets produced at high temperature (which require no heat treatment as they have no tendency to bolt), while sets produced at lower temperatures show no high mortality during or after high-temperature storage. The reduction in bolting following high-temperature storage is not due to water loss by the sets. High temperature of storage throughout or for the last 8 weeks delayed ripening of the bulbs, and resulted in larger yields; no such effect was obtained by high-temperature storage during the first 8 weeks.