Resistance to the anthelmintic levamisole is common amongst most genera of trichostrongylid nematodes of sheep, but is relatively rare in the species Haemonchus contortus. The level of resistance in at least one strain of this species varies during the life cycle. Investigation of this phenomenon, especially in parasitic stages of the parasite may reveal information on resistance mechanisms and factors affecting the selection of resistance. Compared with adult worms of the same resistant strain, immature worms are more susceptible to levamisole both in in vitro contraction assays and an in vivo efficacy experiment. To ascertain if immature parasitic worms of a susceptible strain could be selected for resistance, 3 lines of worms were selected with levamisole for 9 generations. Lines were selected with the same dose rate (2 mg kg-1) at the 4th and 24th day of infection. In addition, because levamisole is less active against immature worms a further line was selected at the 4th day with a higher dose (8 mg kg-1) but similar efficacy to the selection at day 24. In in vitro assays the line selected at day 4 with the lower dose of levamisole showed no resistance compared with an unselected line. The other 2 lines developed high levels of resistance indicating that resistance can develop after selection at either stage. In addition these data directly show that selection pressure is important in determining the rate of development of resistance.