Control of ovine haemonchosis, which is an endemic tropical and sub-tropical helminth disease of considerable economic importance, relies on anthelmintics and grazing management. Practical problems with grazing management, the occurrence of anthelmintic resistance and the cost of anthelmintics have directed attention to possible alternative control strategies. One such strategy is the use of host genetic resistance, a phenomenon known to exist for many years but which has received close attention only within the last twenty years. Within that period the Red Maasai sheep breed in Kenya was shown to be remarkably more resistant to haemonchosis than a number of other breeds. The studies described in this thesis were initiated to confirm this resistance with a view to exploring the prospect of using host resistance to control the disease in the country.