In the hybrid seed population of rice, the variation of germination speed was proved to be determined, at least, by two factors, i. e., one in covering layer derived from mother plant and another in embryo. And the expression of segregation in the latter factor was found to be suppressed considerably by the former. In connection with these facts, the heritability of germination speed by F₃-F₄ regression, and environmental correlation between germination speed and days to heading were preliminarily estimated to be significant.