The acquisition and intensity of infection with gastrointestinal nematodes and with Oestrus ovis are compared over 2 years in sheep grazed intensively (26-36 sheep ha-1) or non-intensively (12 sheep ha-1) in the winter rainfall Overberg region of South Africa. Sheep grazing intensively on grass/legume pastures, spray irrigated in summer, acquired massive mixed infections of Trichostrongylus, Haemonchus and Teladorsagia. Infections were fatal for sucking lambs. Lambs from ewes grazed non-intensively on dry land lucerne in winter became infected with Nematodirus and then with Teladorsagia and Trichostrongylus spp.; resistance to Nematodirus was apparent after 12 weeks; spontaneous cure of Teladorsagia began in hoggets grazing on safe wheat stubble pasture; but Trichostrongylus survived in the sheep. Helminth-induced host mortality was not observed on the non-intensive farm. The efficacy of various methods of control was examined in the intensively managed sheep. Removal from pasture, mixed grazing (cattle before sheep) and an albendazole slow release bolus all significantly (P < 0.05) reduced levels of infection in animals so treated, compared with control sheep which were treated periodically with anthelmintics as standard farm management practice. Removal of sheep from the pasture for 30 days in the summer seemed to have beneficial effects. Treatment of sheep weekly with albendazole increased levels of infection in the sheep over controls. The status of vaccination is described and the potential of intergenic competition in grazing management and perhaps vaccination is discussed. Resistance of nematodes to anthelmintics was relatively wide-spread. In both intensive and non-intensive systems. Oestrus ovis larvae were present in flock sheep for 10-12 months per year. Imagines failed to develop from 27 April-9 August and it was concluded that larvae had to overwinter in sheep to survive. Recommendations for integrated control are made for both intensive and non-intensive systems.