This thesis aimed to identify and parameterise the factors affecting host susceptibility to parasitism. The model system chosen was the caecal nematode Heterakis gallinarum, infecting the ring-necked pheasant Phasianus colchicus. It has become increasingly important to examine the characteristics within hosts that affect susceptibility to parasitism as current control methods are becoming more restricted. Non-invasive parasite control procedures for game birds could solve many of the problems associated with largescale anthelmintic usage and resultant resistance. The research was undertaken using pheasants naive to parasite infection that were orally challenged with H. gallinarum eggs, or using individuals previously exposed to parasitism that were again naturally exposed. The results showed that host susceptibility to parasitism was affected by variation in the T-cell mediated immune response of the pheasant host, possible acquired immune resistance of the pheasant to H. gallinarum parasitism, nutritional stress interacting with body condition, and possible trade-offs between condition, spleenic response, secondary sexual ornaments and H. gallinarum parasitism. H. gallinarum did not seem to affect pheasant morbidity subsequent to parasite challenge.