Studies were carried out on possible control measures using different drugs or management procedures which could help in the control of anthelmintic resistance in sheep nematodes, particularly that of ivermectin (IVM) resistance in Haemonchus contortus. Other aspects such as selection for IVM resistance, the effect of IVM on susceptible and resistant larval stages of H. contortus and the survival of their free-living stages under laboratory or field conditions were also investigated. In addition, a computer model to simulate the development of anthelmintic resistance in breeding ewes was examined incorporating Brazilian field data. Isoenzyme profiles of susceptible and resistant strains were also analysed.