Japonica-Indica hybrid rice lines produced tillers vigorously during the 30-day period after transplanting and exhibited a high degree of heterosis in terms of crop growth rate (CGR). Though the degree of heterosis decreased thereafter, it rose again after heading, contributing to the good characteristics of maturation in hybrid rice. High yield of hybrid rice lines was attained by the summation of the increase of each yield component. Response of hybrid rice to fertilizer was distinctive with the enhancement of both dry matter production and yield associated with the large amount of fertilizer applied. Tolerance to unfavorable environments such as low temperature and low light intensity was higher than that of the parental varieties at the vegetative stage, whereas at the reproductive stage it was lower than that of Japonica varieties. Thus, the Japonica-Indica hybrid rice lines appear to have the potential to achieve a superhigh yield in the northern region of Japan.