Trichostrongylus tenuis is a nematode that lives in the caeca of wild red grouse. It causes disease in red grouse and can cause fluctuations in grouse pop ulations. The aim of the work described in this thesis was to study aspects of the ecology of the infective-stage larvae of T.tenuis, and also certain aspects of the pathology and immunology of red grouse and chickens infected with this nematode. The survival of the infective-stage larvae of T.tenuis was found to decrease as temperature increased, at temperatures between 0-30 C? and larvae were susceptible to freezing and desiccation. The lipid reserves of the infective-stage larvae declined as temperature increased and this decline was correlated to a decline in infectivity in the domestic chicken. The occurrence of infective-stage larvae on heather tips at caecal dropping sites was monitored on a moor; most larvae were found during the summer months but very few larvae were recovered in the winter. The number of larvae recovered from the heather showed a good correlation with the actual worm burdens recorded in young grouse when related to food intake. Examination of the heather leaflets by scanning electron microscopy showed that each leaflet consists of a leaf roll and the infective-stage larvae of T.tenuis migrate into the humid microenvironment' provided by these leaf rolls. Scanning electron microscopy showed that the adult nematodes burrowed into the mucosa as well as lying on its surface and that the caecal mucosa of heavily infected grouse became disrupted in areas of nematode activity. The caecal mucosa of lightly infected grouse exhibited little damage and the caecal mucosa of grouse treated with an anthelmintic and shot 5-6 months later was similar to that of lightly infected birds. Some of the nematodes from these treated birds were covered in rosette-shaped cells which have been tentatively identified as adherent lymphocytes. The cuticle of adult T.tenuis was superficially annulated but did not possess cuticular ridges, as described in some other trichostrongyle nematodes. Primary and challenge infections with T.tenuis were established in the domestic chicken and these reached patency but nematodes were expelled in blood-stained balls of mucus and all adult nematodes had been expelled from the birds 30 days after dosing with infective-stage. Following trickle doses of larvae, there was a rise and then a fall in nematode egg output but larvae administered later in the trickle infection appeared to fail to establish. Light and scanning electron microscopy showed haemorrhagic lesions and blood spots on the caecal mucosa of infected chickens and nematodes were found to burrow beneath mucus secreted on the mucosal surface. There were significant increases in the proportions of circulating leucocytes in infected chickens, but only on certain days of infection. No antibodies to T.tenuis were detected in the blood of infected chickens. There was a decrease in the length of the caeca of infected chickens during the period when the Ill nematodes were being expelled from the caeca. Nematode egg output continued to rise during an infection in young red grouse and there was no expulsion of nematodes from these infected birds. Infective-stage larvae that had been attenuated by cobalt 60 irradiation stimulated some degree of resistance to challenge infection in the domestic chicken but not in the red grouse. It was concluded that immunization with irradiated larvae would be of little use in the control of T.tenuis in red grouse.