Lambs infected with two isolates, one British and one American, of Haemonchus contortus were treated with increasing doses of ivermectin. Eggs from the highest dose that had not eliminated the infection were cultured and larvae used to infect another lamb. After three generations the H. contortus was resistant to 0.2 mg/kg ivermectin. The results stress the ease with which ivermectin resistance can be selected if high selection pressure is applied. (c) 2005 Elsevier B.V. All rights reserved.