The toxicity of tetrahydrofuran lignan grandisin was evaluated against larvae of Chrysomya megacephala F. (Diptera: Calliphoridae). The bioassay involved topical treatment on larvae, topical treatment on egg masses, and incorporation in the larval diet. Grandisin showed inhibition of postembryonic development by ovicidal (30%) and larvicidal (38%) effects and reduced larval weight (4 mg), when topically applied on egg masses and starving larvae (L1) at a concentration of 100 g/ l. These Þndings elucidated the effect of grandisin on the C. megacephala life cycle and its potential to control C. megacephala populations.