Over a period of two years 180 pork pigs, 144 baconers and 144 sows from two slaughterhouses in northern England were examined for gastrointestinal parasites. Sows showed the highest level and intensity of infection, 85 per cent harbouring the dominant parasites Oesophagostomum dentatum and O quadrispinulatum; the highest worm burden recorded was 21,000. Hyostrongylus rubidus was present in 28.5 per cent of sows but virtually absent from younger pigs. Small numbers of Ascaris suum and Trichuris suis were recorded, 16 per cent and 23 per cent respectively in pork pigs and lower in older animals. Levels of parasitism appeared to be influenced by a number of management practices, in particular the housing system.