To study differential nucleus-cytoplasm interactions between the two cultivated rice species, Oryza sativa and O. glaberrima, cytoplasmic substitution lines were made by using a glaberrima strain (G) and an Indica strain of sativa (S). The G cytoplasm had no adverse effect on pollen development when combined with the nucleus of S. On the other hand, when the S cytoplasm was combined with the G nucleus, the substitution line showed no seed set because of male sterility although the pollen grains were normally stained with I2-KI solution. A dominant gene derived from S strain seemed to cause anther indehiscence in the substitution line. Further, a restorer gene (Rf j)from Akebono of Japonica type was effective on pollen restoration in the male sterile line, suggesting that the S cytoplasm is the same as those of Japonica type in terms of a fertility-restoring system.