A female-sterile rice FS-1, derived from the progeny of a backcross of F1 of Fujisaka 5 (Oryza sativa subsp. japonica, FJ) and Tjina (Oryza sativa subsp. indica) with FJ were used for histological study of megagametophytic abortion. FS-1 has a very low spikelet fertility (about 3%) and was regarded to have no true embryo sac. Paraffin sectioning was adopted to study the megagametophytic development of FS-1 compared with its maternal parent FJ. The materials were cultivated in field condition in Tsukuba, Japan and sampled at the middle stage of the panicle development. The results obtained were as follows: (1) FJ showed a normal megagametophytic development and resulted into a mature megagamete while most of the embryo sac of FS-1 underwent megaspore abortion. The remains of the embryo sac and the degenerated surrounding cells filled up the space where the normal embryo sac was expected to exist. (2) Abortion took place when the functional megaspore was being formed. Both the three nonfunctional megaspores near the micropyle and the functional megaspore farthest from the micropylar end were degenerated. No two-nucleus, four-nucleus and eight-nucleus embryo sac were found in the present study. It is concluded tentatively that the megagametophytic abortion of FS-1 occurred when the functional megaspore was being formed.