The effects of modifying drug administration and food intake upon the efficacy of fenbendazole against resistant isolates of Teladorsagia (Ostertagia) circumcincta were investigated using naturally infected sheep and goats. Administration of the manufacturer's recommended dose (MRD) as two 2.5 mg kg(-1) bodyweight doses divided by a 12-hour interval resulted in an apparent increase in efficacy of over 28 per cent compared with conventionally treated sheep. Withholding feed for 24 hours before treatment at the MRD resulted in respective increases of 39.7 per cent (P<0.05) and 25.2 per cent compared with conventionally treated sheep and goats. A combination of food withdrawal and divided dosing did not further increase treatment efficacy in goats. It is suggested that withholding feed, divided-dosing or combining these methods may extend the useful life of the broad spectrum class of anthelmintics.