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(1) The numbers of species of insects feeding on bracken (Pteridium aquilinum(L.) Kuhn.) were recorded at 213 sites at different altitudes throughout mainland Britain. Herbivore species richness and abandance were also recorded in five detailed altitudinal transects of the North Yorkshire Moors. (2) Only one species, the delphacid bug Ditropis pteridis (Spin.), had a distribution that was clearly and consistently linked with altitude. As altitude increased, it occurred at fewer sites and became less abundant at occupied sites. (3) Ditropis was introduced in two consecutive years onto bracken above its normal altitudinal range. It reproduced successfully over the summer, but failed to overwinter, possibly because it could not synchronize its life-history with delayed emergence of bracken fronds in the spring at high altitudes. (4) Unlike the results of other similar studies, the total number of species of bracken herbivores did …