The transmission of Oesophagostomum species in sows kept on pastureland on a commercial farm was studied over a period of almost two years. Worm eggs were excreted on to the pasture continuously but they gave rise to infective larvae during the summer and autumn only when climatic conditions were conducive to development. The output of worm eggs was maintained at a moderate level and the numbers of adult worms in culled sows rarely exceeded more than a few thousand but some sows carried heavy infestations of immature worms. No direct relationship between the level of the herbage infestation and the numbers of adult worms and the output of worm eggs was apparent. The use of anthelmintics had only a transitory effect on the level of infection and an alternative system of anthelmintic treatment is suggested.