The cDNA of the starch-branching enzyme I gene (IbSBEI) in the sweet potato (Ipomoea batatas) has been cloned and sequenced. The IbSBEI amino acid sequence was 81% identical to that of potato StSBEI. DNA gel-blot analyses demonstrated that at least two copies of IbSBEI are present in the sweet potato genome. IbSBEI was strongly expressed in tuberous roots. Transcript levels in the roots of single leaf cuttings were extremely low during the first 15-40 d after planting and continuously increased up to 50 d, by which time the tuberous roots had almost completely developed. This indicates that IbSBEI may work in concert with the AGPase large subunit during the primary phase of starch granule formation.