The effect of aqueous and alcoholic extract of the leaves of Ocimum. sanctum was studied on thespontaneous movements of the whole worm (w.w) preparation and nerve muscle (n.m.) complexof Setaria cervi (S. cervi) and on the survival of microfilariae (m.f.) in vitro. Both the extractscaused inhibition of the spontaneous motility of the w.w. and n.m. complex of S. cervicharacterized by initial stimulation followed by reversible paralysis, aqueous extract at a higherconcentration showed immediate effect and irreversible paralysis. The concentration required toinhibit the movements of n.m. complex was 1/4th for aqueous and 1/3rd for alcoholic extractcompared to that for the w.w., suggesting a cuticular permeability barrier. On the m.f. the lethalconcentration (LC 50 and LC 90) were 35 and 50 ng/ml for aqueous whereas, 60 and 85 ng/ml foralcoholic extracts respectively.