The interrelationship of Bacillus thuringiensis Berliner var. kurstaki to the diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), and its primary parasitoid, Diadegma insulare Cress. (Hymenoptera: Ichneumonidae), was investigated using laboratory colonies of the insects. The differential response of third-instar diamondback moth, parasitized and unparasitized, to B. thuringiensis (Bt), and the ability of D. insulare to oviposit in Bt-stressed hosts were determined. No significant difference (P gt 0.05) was found between the mean mortality of parasitized and unparasitized larvae at each of three concentrations (154, 334, and 2,237 IU/ml) of Bt endotoxin. The three concentrations were equivalent to the LC-30, LC-50, and LC-90 of Bt potency based on preliminary tests. Parallel line assay analysis, however, revealed that the linear dose-response regressions of parasitized and unparasitized larvae were highly significant (P = 0.0001). The LC-50s of parasitized versus unparasitized larvae were 373 and 175 IU/ml Bt endotoxin, respectively, indicating that parasitized larvae were less susceptible to Bt. Female D. insulare oviposited in Bt-stressed hosts. The percentage of D. insulare females emerging from Bt-treated larvae (41.4%) was not significantly different from that of untreated larvae (32.0%).