Larvae o f the arctiid moth Tyria jacobaeae reared on Senecio jacobaea or 5. vulgaris take up and store pyrrolizidine alkaloids (PAs) from their host plants. Individual PAs are taken up without preference. The PA patterns found in the insect bodies correspond to the PA com posi tion o f their host plants. Like plants the insects store PAs as N-oxides, and larvae as well as pupae are specifically able to N -oxidize any tertiary PA. Callimorphine ( 0 9-(2-methyl-2-acetoxybutanoyl)-retronecine), an insect PA well known from several arctiids, was found in pupae and imagines o f Tyria which as larvae had been fed on S. jacobaea. It is accompanied by small amounts o f its isomer 0 7-(2-m ethyl-2-acetoxybutanoyl)-retronecine named isocallimorphine. The callimorphines may well account for 45% o f total PAs found in the insect. Only small amounts o f callimorphine were detected in pupae o f Tyria which as larvae had been fed on S', vulgaris. [l4C]Callimorphine N -oxide was isolated and identified from Tyria pupae which as larvae received [14C]retronecine. It is suggested that Tyria is able to esterify retronecine, derived from hydrolysis o f ingested plant PAs with a necic acid produced by the insect. During metamorphosis the formation o f callimorphine is restricted to the early stage o f pupa tion.