The effect of natural day length on heading time in Japanese rice (O. sativa L.) cultivars has not yet completely been made clear, because of the difficulty in eliminating the effect of temperature. Five cultivars were grown under two constant day lengths of 20 and 50 minutes shorter than the day length at the summer solstice, 15 hours and 5 minutes. These day length corresponded to those of July 20 th. and August 10 th. respectively. Growth duration from seeding to heading in those day length was shorter than that in the day length at around the summer solstice.