The persistent anthelmintic effect of ivermectin given subcutaneously at 200 µg/kg was evaluated against induced infections of Haemonchus placei, Ostertagia ostertagi, Cooperia pectinata, C. punctata, Bunostomum phlebotomum and Oesophagostomum radiatum in cattle. Forty-four Friesian bull calves raised under worm-free conditions were restrictively randomized to one untreated control group and 3 ivermectin treated groups of equal size according to mass. Animals in the different treated groups were treated either 9, 7 or 5 d before infection, which was induced in all animals on the same day. Nine days after treatment the effect of ivermectin was virtually undiminished against O. ostertagi and B. phlebotomum and 7 d after treatment against Cooperia spp. Counts of all worms were reduced by 99% or more following the treatment given 5 d before infection. According to the non parametric method, 80% efficacy in 80% of the treated animals were achieved against all 5 worms up to 7 d after treatment and against O. ostertagi and B. phlebotomum up to 9 d after treatment.