A susceptible strain of Heligmosomoides polygyrus was selected for 15 generations with increasing doses of ivermectin (0-6 mg/kg). A passage strain was developed, parallel with the ivermectin-selected strain, to control for changes due to rapid passage from mouse to mouse. The LD50s of the 8th and 15th generations of the ivermectin-selected strain were 1.5 times that of the susceptible strain. The LD50 of the passage strain at generations 8 and 15 remained similar to that of the susceptible strain. Ivermectin efficacy was lower against the LA stage than against the adult stage in the susceptible strain, the Ivermectin-selected strain and the passage strain at generation 8.