Increasing reports of failure of synthetic pyrethroids for the control of Bovicola ovis on sheep lead to a preliminary investigation into the insecticide resistance status of this pest during 1994. Eighteen strains of lice from North and South Island flocks experiencing failure of synthetic pyrethroid treatments to control lice were exposed to cypermethrin (high-cis) in a treated surface (contact) bioassay. Lice from a group of hermit sheep were chosen as a reference susceptible strain against which resistance of field strains was measured and resistance factors (RFs) calculated. Low to moderate resistance to cypermethrin (high-cis) was demonstrated, with RFs ranging from 1.0 to 12.4.