The main objective of this study was to evaluate the anthelmintic activity of Adhatoda vasica (Acanthaceae) in vitro against the gastrointestinal nematodes of sheep. The aqueous and ethanolic extracts of Adhatoda vasica aerial parts were evaluated by egg hatching and larval development assays. The aqueous and ethanolic extracts at 25-50 mg/ml concentrations exhibited ovicidal and larvicidal (p<0.05) activity against gastrointestinal nematodes. The plant extracts showed dose-dependent inhibition (P<0.05). The ethanolic extract at the concentration of 50.0 mg/ml was more effective in inhibiting egg hatching and larval development of gastrointestinal nematodes. The effective dose (ED₅₀) of aqueous and ethanolic extracts were determined graphically from linear regression equation with probit scale, y = 5. The results of this study suggested that Adhatoda vasica extracts may be useful in the control of gastrointestinal nematodes of sheep.