Recurrent backcrossing has been carried out with a view to transfer a gene for non-glutinous endosperm from two strains of O. glaberrima (Wxg/Wxg) to glutinous japonica and indica varieties (wx/wx) of Oryza sativa. In the course of backcrosses Wxg/wx segregants were crossed with each of the two glutinous varieties of sativa as the respective recurrent male parent. The wx/wx and Wxg/wx segregants in the successive generations were consistently fully fertile and semi-sterile, respectively. The semi-sterility of Wxg/wx plants was attributable to abortion of most of the pollen grains carrying the gene wx. The nucleus but not cytoplasm was related to the semi-sterility. The Wxg/Wx plants having the gene for non-glutinous endosperm of a glaberrima strain and a japonica variety of sativa were also semi-sterile. Both wx- and Wx-megaspores in the plants heterozygous for the gene Wxgwere deleteriously affected. The results could be explained by assuming that a factor tightly linked with the gene Wxgof glaberrima sterilizes gametes not carrying it in the heterozygotes and that the gametocidal action is exerted when combined with the sativa nucleus by the recurrent backcross method.