A pyrrolizidine alkaloid was isolated from the stems of Parsonsia laevigata ALSTON, a plant upon which the larvae of Idea leuconoe ERICHSON feed, and its structure was determined to be 1- (2 'S,3 'S )-2 ',3 '-dihydroxy-2 '-isopropylbutanoyloxymethy1-5,6-dihydropyrrolizin-7-one.