1. The present experiments were carried out as a part of the cooperative research undertaken by the working party on rice breeding of the International Rice Commission, to make clear the fundamental characters concerning the earing time of the rice plant. 2. Twenty varieties supplied by 8 countries were grown from their earliest seedling stages under 4 different day-length conditions, viz. 10.5 h., 11.5 h., 12.5 h. and the natural day-length. 3. Vegetative growth periods under the optimum day-length were 40-60 days for Japanese varieties, 40-70 days for Burmese varieties, 50-80 days for Indian varieties and 70-150 days for Javanese varieties. 4. The longer limit of the optimum day-length was about 12 hours for Japanese varieties, and it was clearly shorter for the southern varieties with two exceptions, Heenati (Ceylon) and Gendjah Ratji 277 (Java). 5. Only the above exceptional varieties could produce the ear under the natural day length of Tokyo. 6. Under the optimum day-length, the varieties of longer vegetative growth period produced more leaves in the main stem, and a longer day-length than the optimum increased the number of the leaves, with the exception of Javanese varieties of very long growth period.