Number of days from heading to harvest of indica rice in the tropics is considerably less than that of japonica in Japan. The present study revealed that such a difference is due to the former rice having genetically less days required for ripening of grains than the latter. Photosynthetic activity, total nitrogen and soluble protein contents in the flag leaves of indica also declined more rapidly than japonica. Since short ripening period may be unfavorable for grain production, prolonging it by breeding or improving cultivation techniques may be one of the methods for increasing yield of indica rice.