The microbial insecticide bacillus thuringiensis Berliner var. kurstaki is used to control forest pests in regions where tansy ragwort, Senecio jacobaea L. occurs. Biological control of this noxious weed may be compromised if the cinnabar moth, Tyria jacobaeae (L), is susceptible to B. thuringiensis. In laboratory feeding tests, it was found that foliage dipped in B. thuringiensis solutions had very little effect on survival of first, second, and third instars, but fourth and fifth instars were susceptible. Field tests were done to test the effect of B. thuringiensis on larval defoliation of potted tansy ragwort plants at five different application rates. The application rate significantly affected percentage of leaves defoliated and mean number of cinnabar moth-days per plant. In both control plants and those treated with B. thuringiensis, defoliation by cinnabar moth did not affect plant biomass within the 10-d course of the experiment.