Spatial distribution is understood to connote distribution of density (including zero density) over the land surface at a given time. Combining fact and conjecture, the following is a theory of spatial distribution of Ixodes ricinus in Britain. The tick passes practically the whole of its 3-year life span on the ground, spending a total of only 3 weeks on hosts. A considerable amount of vegetational cover is necessary for efficient survival on the ground; the more important limiting circumstances in lesser cover seem to be humidity (conditioned by temperature) in summer, perhaps temperature (alone) in winter, and the activity of predators. A host is necessary for mating as well as food purposes. Though small at best, the tick's chances of achieving a host are greater the more hosts on the ground. In practice this means the more sheep, since the latter are by far the main hosts. For the most part, spatial distribution to-day can be explained adequately in terms of interplay between amount of ground-cover and of host-potential, with cover the master factor. Thus poor cover keeps the heavily stocked lowland pastures free of permanent tick population even when opportunity of colonization is offered. The tick is confined to the hills and moorlands with their deep vegetation layers. There, within limits, it is more numerous on a pasture the more universal the distribution of adequate cover (because it drops at random from the host) and the higher the host-potential. Among hill and moorland pastures, however, there are some cases where, despite ample cover and hostpotential, either (1) no ticks are present, or (2) the tick population is lower than it ought to be. These cases can be explained on grounds of lack of opportunity for colonization (1); or colonization so recent that population has not attained the level of ‘dynamic equilibrium’ with the environment (2). The question of ‘spread’ (colonization) is discussed fully in the text. The sheep flock is the main agent in spreading ticks. Spread is comparatively slow and relatively small in extent nowadays. There has very probably been considerable increase both in tick population densities and in infested areas since the advent of intensive hill and moorland grazing with farm stock some centuries ago.