There is now a solid body of theoretical work(1-4) demonstrating that the spatial structure of the habitat combined with animal movement strongly influence host-parasitoid dynamics. The spatial pattern over which parasitoid search takes place can be affected by the distribution of the hosts(5), by the spatial arrangement of the host’s habitat(6) and by the spatial scale at which the parasitoid perceives variation in host abundance(7,8). Empirical work, however, has been largely restricted to small-scale field studies of less than one hectare(6,9) with very few larger(10,11). Here we report initial results of a many-year, large-scale study that is among the first to examine the interaction between a population-level process (parasitism) and anthropogenic forest fragmentation at large and at multiple spatial scales. We demonstrate that parasitism by four species of parasitoids attacking the forest tent caterpillar, Malacosoma disstria, is significantly reduced or enhanced depending on the proportion of forested to unforested land. Each of the parasitoid species responds to this mosaic at four different spatial scales that correspond to their relative body sizes, Our data give empirical support to the argument that changes in landscape structure can alter the normal functioning of ecological processes such as parasitism, with large-scale population consequences(3’4).