This paper describes the in vivo anthelmintic activity of Azadirachta indica seeds to justify their use in South-Asia by traditional animal healers. Seeds of A. indica were administered as crude powder (CP), crude aqueous (CAE) and crude methanolic extracts (CME) at the doses of 1 and 3g/kg of body weight to sheep naturally infected with mixed species of gastrointestinal nematodes. The study design also included untreated as well as treated controls. Faecal egg count reduction and larval counts from coprocultures were performed pre- and post-treatments to assess the anthelmintic activity. Crude powder and CME did not show significant activity (P>0.05) at the lower dose used but were found effective at 3g/kg and the maximum anthelmintic effect was observed at the 15 days post-treatment with both crude powder and CME (P<0.01) with a maximum reduction of 29.3% and 40.2%, respectively in eggs per gram of faeces. Haemonchus contortus and Trichostrongylus species were found susceptible (P<0.01) to higher doses of CP and CME of A. indica. However, CAE did not exhibit any considerable reduction in EPG as well as larval counts. Levamisole (7.5mg/kg), a standard anthelmintic agent, exhibited 99.2% reduction in EPG (P<0.001). Though of low efficacy compared with levamisole, the use of A. indica seeds against gastrointestinal nematodes may be justified in some situations, depending on the nature and intensity of the helminth infections.