Effects of day and night temperatures on flower-bud formation and bolting of Japanese bunching onion (Allium fistulosum L.) were investigated by using mid-season flowering 'Kincho' and 'Asagi-kujo', and late-season flowering 'Cho-etsu'. Exp.1. Plants were grown at 3, 7, 11, and 15°C at night and 20°C during the day for 20 to 60 days. Flower-bud formation was promoted at 3°C and, especially, at 7°C in all cultivars. More than 50% of 'Kincho' and 'Asagi-kujo' plants formed flower buds at 15°C when grown for 60 days ; only 10% of 'Cho-etsu' bolted. Exp.2. Plants were grown at 7.0, 13.5, 20.0 or 26.5°C during the day and 7.0°C at night. Bolting of 'Kincho' and 'Asagi-kujo' plants at 7.0°C and 26.5°C was slightly retarded, compared to plants exposed to 13.5°C and 20.0°C. The bolting rate of 'Cho-etsu' was lower in all day temperatures than were those of 'Kincho' and 'Asagi-kujo', irrespective of the duration of treatments. Over 60% of 'Cho-etsu' bolted when grown at 7.0°C and 13.5°C for 75 days, whereas only 33.3% bolted at 20.0°C ; no plants bolted at 26.5°C. Thus, 'Cho-etsu' plants were devernalized at day temperature 20.0°C and 26.5°C. Based on these results, the optimum night temperature for flower-bud formation of Japanese bunching onion is 7°C at 20°C during the day. Devernalization occurs in 'Cho-etsu' at and above 20°C ; it rarely occurs in 'Kincho' and 'Asagi-kujo' below 26.5°C.