<jats:p>Some 250 substances were assessed for larvicidal activity against the free-living stages of <jats:italic>Haemonchus contortus</jats:italic>. Most of them were either plant products or chemical compounds found in plants. <jats:p>Results indicate that promising chemical types worth further investigation are ketones of low molecular weight, alkyl di– and trisulphides, and organic <jats:italic>isothiocyanates</jats:italic>. Compounds whose molecular structures include the allyl group (e.g. anethole) and the methylenedioxy ring system are potentially active, safrole being particularly effective. Alkaloids as a group were not very effective, but eserine (physostigmine) was outstandingly active, and nicotine sulphate and arecoline hydrobromide showed pronounced activity. Other substances worthy of mention were phenyl mercuric phthalate, galegine sulphate, 8-hydroxy quinoline, methyl salicylate, 10-allyl phenothiazine, guaiacol, thymol, <jats:italic>l</jats:italic>-limonene and sabinol. <jats:p>Most of the plant products tested were essential oils: these generally were active, the lowest activity being noted in those oils rich in sesquiterpenes. Activity in the essential oils could be correlated with that of the known compounds.