Two new pyrrolizidine alkaloids (PAs) were isolated from adults of Creatonotos transiens, the larvae of which had ingested retronecine or ester alkaloids. The structures were elucidated by GC-MS and 1H and 13C NMR spectroscopy as 09-(2-hydroxy-3-methylpentanoyl)-retronecine (creatonotine) and its isomer O7-(2-hydroxy-3-methylpentanoyl)-retronecine (isocreatonotine). The 2-hydroxy-3-methylpentanoic acid obtained by hydrolysis of the creatonotines is the 2S,3S-stereoisomer. The creatonotines as well as accompanying trace amounts of callimorphine were shown to be synthesized by both sexes from dietary retronecine by esterification. If Creatonotos larvae had been fed with Gynura scandens as PA source, the insect PAs accounted for about 75% of total PAs isolated from adults, indicating that Creatonotos degraded plant PAs and subsequently re-esterified the resulting retronecine. This work provides definitive proof that an arctiid moth is able to hydrolyse plant ester alkaloids and re-esterify the resulting necine base with acids which are intermediates of insect metabolism. PAsisolated from Creatonotoswere exclusively present in the form of their N-oxides. © 1991.