Two studies were carried out in order to test the effects of neem tree extracts (Azadirachta indicaA. Juss) on sheep bot fly larvae (Oestrus ovisL. Diptera: Oestridae). First, aqueous extracts from neem seeds (ASNE) at 0, 5 y 10% (w/v) concentrations were tested on larval mortalityin vitro. In a second study, the effect of oral administration with neem seed meal (0, 100 y 200 mg/kg) and neem leaves (1% of diet) on number of larvae found at necropsy and larval development was evaluated in experimentallyO. ovis-infected sheep. Results in Experiment 1 showed a significant (P < 0.05) effect of ASNE on time to L1 mortality in a dosis-dependent manner. In Experiment 2, oral administration of seeds or leaves did not affect the number of larvae found at necropsy of the sheep, but interfered with larval development and there was a tendency to reduce larval weight at the end of the infection period (55 d).