The ultrastructural changes in the intestines of Ascaris suum and Syngamus trachea, induced by in vivo treatment of the hosts with the anthelmintic mebendazole, are reported. The primary site of drug action seemed to be the organelles involved in the secretory mechanism of the intestinal cells. The block in the transport of secretory granules and in the movement of other subcellular organelles coincided clearly with the disappearance of cytoplasmic microtubules. On the other hand, the microtubular system of the host cells was unaffected by the treatment. Degenerative changes in the intestinal cells of the parasites observed afterwards were correlated with the primary deteriorative effect of the drug on cytoplasmic microtubules.