SummaryThe long term efficacy of an irradiation attenuated larval (L3) vaccine againstStrongylus vulgariswas tested in ponies which were reared on pasture. Prior to foaling, mares were divided into two groups. One group of mares and foals received regular (eight weekly) treatment with ivermectin and the second group remained untreated. Half the foals in each pasture group were vaccinated at eight to ten weeks of age. Foals were weaned at three to four months of age and maintained on separate pastures. At eight to ten months of age, ponies were placed in box stalls and half of each treatment group were challenged withS. vulgaris(5 × 1000 L3). Clinical signs and lesions typical of acute verminous arteritis were found at necropsy in the ivermectin treated non‐vaccinated challenged yearlings. Ivermectin treated vaccinated challenged yearlings did not show these clinical signs, had markedly reduced to absent arterial lesions and showed an 89 per cent reduction in arterial larval burdens post mortem. Significant differences in clinical signs, arterial lesions or arterial larval burdens were not seen between vaccinated and non‐vaccinated foals reared without benefit of ivermectin treat