The epidemiology of the gastrointestinal nematodes of farmed red deer was followed over three years on two farms in north Hertfordshire. Worm counts on 10 animals (four months to 10 years old) between November and March showed that mixed ostertagid infections were dominant with very small numbers of Trichostrongylus axei, Cooperia punctata and Oesophagostomum species also present in some individuals. An increase in the faecal nematode egg count of the hinds in summer was followed by an increase in pasture larval counts which peaked between September and November. Many calves had patent infections by the first week of September. Although the pasture larval counts were very low at the beginning of the 1989 season, anthelmintic treatment at turnout in May had little influence either on the summer increase in faecal egg count or on the subsequent pasture larval counts, and repeated treatments had only a moderate influence. However, the movement of one of the calving groups to an aftermath in mid-July did reduce the infection to which their calves were exposed. Nevertheless, none of the calves showed significant effects of parasitism when they were removed from the pasture and treated at weaning in early September.