The effect of natural selection and of competition were studied by the use of five artificial mixtures of rice varieties. Each mixture was made by mixing equal amount of seeds of two contrasting varieties, the Japanese lowland and the Japanese upland ones. These five mixtures were grown under three conditions, Iowland, upland ordinary and upland nursery, for one to three years (Table 1). The proportion of upland variety increased substantially after only one year of cultivation in all mixtures under the two upland conditions. These increases were slightly higher undr the upland nursery than the upland ordinary condition (Fig. 1). The sirnilar increase of upland variety also occurred to a lesser extent in the three out of the five mixtures. under the lowland condition. A comparison of the five mixtures confirmed that the survival of upland variety was infiuenced not only by the growing condition but also by the lowland variety combined. The mixtures planted under the lowland condition were grown repeatedly under the same condition for three years. The increasing trend of upland variety varied in each mixture the increase was moderate but clear in some mixtures while it was not clear in the others (Fig.2).