Four pyrrolizidine alkaloids (PA) were isolated from Ageratum houstonianum and their structures elucidated by spectroscopical methods. Besides the already known lycopsamine three new PA were found. Their structures are the 2S-2-hydroxy-2,3-dimethylbutanoyl-O-9 as well as the O-7 esters of retronecine and the O-9 derivative of heliotridine. (C) 2001 Elsevier Science Ltd. All rights reserved.