One of the most remarkable features of parasitic infections in general, and Ostertagia circumcincta infection of sheep in particular, is the extensive variation among hosts in resistance to infection, as assessed by parasite burdens and production of eggs or infective larvae. Here Mike Stear, Michael Park and Stephen Bishop describe the factors that account for the variation among animals within a flock, including dam, sire, sex, date of birth and history of exposure to infection. There are no detectable genetic effects in lambs less than three months old. Genetic control of an acquired response develops in two stages: first, a reduction in the average egg production per worm, which is associated with the development of a parasite-specific local IgA response; and second, control of worm burden, which is associated with the production of globule leukocytes in the abomasal mucosa.