Many rice mutants with altered storage protein composition, which relate to protein body II (PB-II), have been produced in order to obtain rice with new cooking properties or lowered protein digestibility, but the PB structures in the endosperm of such mutants remain obscure. We examined the polypeptide composition and ultrastructure of the endosperm during seed development using rice mutants with the low-glutelin-content gene (Lgc1) and/or 26 kDa globulin deficient gene (glb1). Lgc1 and glb1 have deletions of a region including glutelin structural genes and 26 kDa globulin gene, respectively. Mutants with Lgc1 accumulated prolamin polypeptides from an early stage of seed development, and numerous intact PB-I and a few, small, chipped PB-II were observed in the endosperm. The glb1 mutant showed deformed PB-II structures, which had similar electron density to wild-type PB-II. Both glutelins and globulins accumulate in PB-II, but the influence of the change in their composition on PB-II structure would be different. The unique PB-II structure in the glb1 mutant may bring about new processing properties for rice.