The benzimidazole compounds albendazole (ABZ) and triclabendazole (TCBZ) are both effective against Fasciola hepatica, although ABZ is only effective against adult flukes. Additionally, ABZ is a broad-spectrum nematodicidal compound with well-known ovicidal activity. However, no data on the ovicidal effect of TCBZ against F. hepatica eggs are available. The work reported here evaluated the comparative ovicidal effect of ABZ, TCBZ and their sulphoxide metabolites on F. hepatica eggs recovered from bile of sheep artificially infected with either a TCBZ-susceptible (Cullompton) or a TCBZ-resistant (Sligo) isolate of F. hepatica. Additionally, the effects of different non-flukicidal methylcarbamate benzimidazole compounds on the hatching of F. hepatica eggs were evaluated. Eggs (500 eggs/mL, n=4) were incubated for 12h either with TCBZ, TCBZ sulphoxide (TCBZ.SO), ABZ (5, 10 and 20 nmol/mL) or without drug (untreated control) (Experiment 1). Additionally, the effect of TCBZ and TCBZ.SO (5 nmol/mL) on egg hatchability was examined after a long (15 days) drug exposure (Experiment 2). Furthermore, the ovicidal effect of ABZ and ABZ.SO at different concentrations (5, 1, 0.5, 0.1 and 0.05 nmol/mL) (Experiment 3), and the effect of fenbendazole (FBZ), oxfendazole (OFZ), mebendazole (MBZ), flubendazole (FLBZ) (5 nmol/mL) and reduced-FLBZ (R-FLBZ) (2 microg/mL) on fluke eggs, were evaluated after a 12-h exposure (Experiment 4). Egg hatch was assessed by direct microscopic observation after incubation at 25 degrees C for 15 days. TCBZ and TCBZ.SO did not affect egg hatch after a 12-h incubation. A similar result was obtained after a much longer drug exposure (15 days) (Experiment 1 and 2). However, a significant (P<0.05) inhibition of egg hatch was observed in ABZ- and ABZ.SO-incubated eggs (Experiments 1 and 3). Additionally, the non-flukicidal compounds (Experiment 4) affected egg hatchability, particularly FLBZ and R-FLBZ. In conclusion, ABZ and ABZ.SO had a clear inhibitory effect on egg development of F. hepatica. However, the most extensively used flukicidal compound, TCBZ, and its main sulphoxide metabolite, did not affect egg hatch, even in TCBZ-susceptible flukes.