Two field cases of apparent inefficacy of albendazole against cestodes in lambs were investigated. In both farms Thysaniezia giardi was identified and 2 critical controlled trials were conducted to determine the efficacy of 5 anthelmintics against T. giardi. Albendazole dosed at 3,8 mg/kg or 7,6 mg/kg live mass, mebendazole at 15 mg/kg were totally ineffective against T. giardi. Treatment with resorantel at 65 mg/kg or niclosamide at 50 mg/kg however, caused expulsion of the strobilae within 16--24 hours after treatment and at autopsy, lambs were free of scoleces of T. giardi.