SUMMARY Prepatent Toxocara canis infections acquired in utero were effectively eliminated in dogs with the use of thiabendazole (tbz) paste. A dosage schedule of 150 mg./kg./day given from the 5th to the 20th or 25th postpartum days was 98 to 100% effective in treated pups. The same dosage given from the 5th to the 10th postpartum days or from the 10th to the 20th postpartum days was less effective. A dosage of 50 mg./kg./day given from the 5th to the 10th or from the 5th to the 25th postpartum days was also less effective. The powder form of tbz had poor anthelmintic efficacy against prepatent T. canis infections.