The transmission of Hyostrongylus rubidus and Oesophagostomum spp. following anthelmintic treatment was studied over a period of two years in a herd of sows kept out-of-doors on a commercial farm in south-eastern England. The sows were moved on to a clean pasture each autumn and at the same time were treated with an anthelmintic. The treatment was repeated six months later when the faecal worm egg count was rising. Contamination of the pasture with worm eggs was both light and intermittent. The pasture herbage remained free of infective larvae until the early summer; subsequently the herbage became lightly infected with larvae so that transmission of the parasites was possible, but limited. The level of infection on the herbage was much lower than was seen in earlier observations when the more commonly used system of treating groups of sows at different times of the year, in between farrowings, was used.