Abstract. Electrophysiological recordings from taste sensilla of the caterpillar Tyria jacobaeae with the pyrrolizidine alkaloids, characteristic compounds from their host plants, demonstrated sensitivity of a pyrrolizidine alkaloid-sensitive cell in the lateral galeal sensilla at concentrations as low as 1 × 10−11 M. Another pyrrolizidine alkaloid-sensitive cell in the medial galeal sensilla responded at higher concentrations. Both pyrrolizidine alkaloid-cells were maximally sensitive to seneciphylline N-oxide and senecionine N-oxide. Seven other pyrrolizidine alkaloids were less stimulating. Monocrotaline N-oxide was the least stimulating. Observation experiments demonstrated that differences in sensitivity to different pyrrolizidine alkaloids at the electrophysiological level were correlated with differences in feeding behaviour; the first feeding bout was of longer duration on diet containing seneciphylline N-oxide than on diet containing monocrotaline N-oxide, and a plain diet was generally not accepted.