Lycoriella ingenua (Dufour) (Diptera: Sciaridae) and Coboldia fuscipes (Meigen) (Diptera: Scatopsidae) are two of the most economically important insect pests of cultivated mushrooms. The toxicities to the fly larvae of the three phenylpropanoids (methyleugenol, myristicin, and safrole) from aerial parts of Asarum sieboldii Miquel (Aristolochiaceae) were compared with those of the currently available carbamate insecticide benfuracarb. In a contactþfumigant mortality bioassay with L. ingenua and C. fuscipes larvae, methyleugenol (1.46 and 2.33 µg/cm²) was the most toxic compound, followed by safrole (2.03 and 2.59 µg/cm²) and myristicin (3.59 and 4.96 µg/cm²), based on 24-h LC₅₀ values. The phenylpropanoids were less toxic than benfuracarb (LC₅₀, 0.75 and 0.55 µg/cm²). In vaporphase mortality tests with the larvae, the phenylpropanoids were consistently more toxic in closed versus open containers, indicating that the effect of the compounds was largely a result of vapor action. Global efforts to reduce the level of highly toxic synthetic insecticides in the agricultural environment justify further studies on A. sieboldii plant-derived products as potential fumigants for the control of mushroom fly populations in mushroom houses and mushroom compost.