A Series of experiments was conducted to examine the relations of planting density, stage of plant growth and fertility level of soil to intra-specific competition and cooperation among rice plants. During the period From planting to earing, there was observed a linear relationship between the logarithm of the density and logarithm of the yield (dry weight per unit area) and an attempt to fit KIRA's equation, Y=Kd EXP [1-a], an empirical relationship beteen density and yield was successful. In regard to the final yield, however, neither to the grain yield nor to the straw yield, KIRA's epuation gave any statistically good fit. In the present instance the regresions of grain yield and straw yield on density were found to be represented by the following empirical formulas, respectively. [numerical formula]