Abstract:Cytoembryologilal studies were carried out to reveal the origin of haploids occurring in the natural population of polyembryonic line SB-1 from Oryza sativa L. The results showed that one or both synergids might develop into embryos prior to fertilization with frequencies of 2.24% and 1.75 % respectively in the observed ovaries. Double fertilization was observed in the embryo sacs with one previously-developed multicellular embryo. In caryopsis 3—8 days after anthesis, 1.62 % of abnormal slender embryos, which were morphologically different from zygotic embryos, were present between the endosperm and nucellus, while 0. 8% of pyriform embryos near the micropylar end without endosperm were observed. These embryos were considered to possess the apogamous origin of unfertilized synergids. Rarely, in addition some ovaries the unfertilized central cells automatically developed into endosperms, which were confirmed by the presence of distinctly intact egg apparatus and the absence of trace of pollen tubes. The occurrence of apogamy in SB-1 and its potentiality to rice breeding are here with discussed.