Abstract.1. 239 species of leafmining insects have been recorded from thirty‐seven species of trees and large shrubs in Britain.2. A significant species—area relationship was obtained for these insects, but it explained only 19% of the variation about the regression he. A further 23% of the variation may be accounted for by taxonomic relationship of host plants.3. A significant correlation was established between the number of species of leafminers and those of mesophyll‐feeding leafhoppers for twentyeight host tree species. Both guilds are dominated by host specific feeders.4. It is suggested that species—area effects are important in determining species richness of phytophagous insects mainly when groups of closely related host plants are considered. With more diverse arrays of hosts, taxonomic influences, which are probably mainly chemical, may outweigh such area related