This study was undertaken to determine the effect of thiabendazole and haloxon on the free-living stages of Haemonchus contortus and to determine the suitability of the free-living stages for their use in primary drug evaluation for chemotherapy or for chemical control of free-living stages. Cleaned unembryonated ova, embryonated ova, first-, second-, third-, and exsheathed third-stage larvae were exposed to suspensions of 5.0%, 1.0%, 0.5%, 0.25%, 0.12%, 0.05%, and water soluble concentrations of each drug for exposure times of 0.25, 0.50, 1.0, 2.0, 4.0, 8.0, and 16.0 hours at pH 7.0 and 25° C. Survival was assessed by observing any subsequent development or motility. Thiabendazole was partially ovicidal to unembryonated ova at water soluble levels and after exposure of only 15 minutes. Exposure of longer than 4 hours was 100% lethal. No other stages were effected by thiabendazole. Haloxon was ineffective against all stages tested.