The influence of parasitism byMicroctonus aethiops(Nees) (Hymenoptera: Braconidae) on the susceptibility of its primary host, the alfalfa weevil,Hypera postica(Gyllenhal) to carbofuran (Furadan®), methoxychlor, methyl parathion and Supracide® was studied in the laboratory. Relative toxicities to the adult and pupal stages of the parasite were also determined.Furadan was the most toxic compound to nonparasitized weevils with an LD50of 1.18 μg/g body wt, followed by methyl parathion, Supracide and methoxychlor. Methoxychlor was about 3000 times less toxic than Furadan.Parasite egg development in the host did not influence weevil susceptibility. However, parasite larval development increased host susceptibility and the LD50values decreased by almost one-half.Supracide, Furadan, methyl parathion and methoxychlor were ca. 2, 3, 5 and 600 times more toxic to adult parasites than to adult weevils.Parasite pupae displayed considerable resistance. Furadan and methyl parathion were about 90 times less toxic to pupae than to adults. Similarly, Supracide and methoxychlor toxicity decreased 54- and 17-fold, respectively.