Amylose and amylopectin of rice mutants deficient in,a starch synthase (SS) isozyme in the endosperm, either SSI (Delta SSI) or SSIIIa (Delta SSIIIa), were structurally altered from those of their parent (cv. Nipponbare, Np). The amylose content was higher in the mutants (Np, 15.5%; Delta SSI, 18.2%; Delta SSIIIa, 23.6%), and the molar ratio of branched amylose and its side chains was increased. The chain-length distribution of the beta-amylase limit dextrins of amylopectin showed regularity, which appeared consistent with the generally, accepted cluster structure, and the degrees of polymerization found at the intersections were taken as the boundaries of the B-chain fractions. The mole % of the B-1-B-3 fractions was changed slightly in Delta SSI, which is consistent with the proposed role of SSI in elongating the external part of clusters. In Delta SSIIIa, a significant increase in the B-1 fraction and a decrease in the B-2 and B-3 fractions were observed. The internal chain length of the B-2 and B-3 fractions appeared to be slightly altered, suggesting that the deficiency in SS affected the actions of branching enzyme (s).