It is usual, when considering the trichostrongyle infections of sheep, to refer to parasitic stages in the host and free-living stages on pasture as two separate entities with ecological, structural and physiological differences emphasized by their temporal relationship. They are, however, interlinked phases in a continuous exchange process in which the host acts as a vehicle. The role of the host as a mediator in this exchange has received little attention until recently and has been mainly restricted to a more dynamic approach to problems of immunity. The behaviour of the host, as well as that of the parasite, is obviously of great importance, but there has been little work on the behaviour of sheep in relation to patterns of helminth infections, although Taylor (1954) discussed the problem in general terms.