Four groups of five Romney lambs were treated by plunge dipping with one of four registered organophosphorus flystrike preventatives. Untreated lambs acted as controls. The sheep were challenged at weekly intervals with larval implants of organophosphate-susceptible and -resistant strains ofLucilia cuprina. All four treatments provided 19–21 weeks protection against susceptible larvae but chlorfenvinphos provided the longest protection (16–17 weeks), followed by propetamphos (15–16 weeks), dichlofenthion (10–13 weeks) and diazinon (9–13 weeks), against the resistant strain.