The infective larvae of Teladorsagia circumcincta have a protective sheath that is lost soon after they reach the rumen of the sheep (the definitive host). Incubation in vitro with 50 mM imidazole caused more than 75% of L3 T. circumcincta to begin exsheathing within 2 hr. The initiation of exsheathing was less likely at pH 6.2 than at pH 7.8. The apparent pKa of this process was 7.08, similar to that for the conversion of imidazolium+ to imidazole. Both the extent and the initial rate of exsheathing initiation increased with imidazole concentration (the apparent K½ was about 50 mM). The initial rate of exsheathing initiation was stimulated by lactose and maltose, but not by some other carbohydrates, and by propylamine and imidazole acetic acid, but not by histidine.