Foliar endophytic fungi appear to be ubiquitous in nature, occurring in a very wide range of herbaceous plants. However, their ecological role within forbs is very poorly known and interactions with foliar-feeding insects virtually unexplored. In this study, leaves of Cirsium arvense were infected with different combinations of endophyte fungi that had been previously isolated from this plant species. Two months later, leaf material was fed to larvae of a generalist insect, Mamestra brassicae, and adults of a specialist feeder, Cassida rubiginosa. Endophytes had different effects on the two insects; one species, Chaetomium cochliodes, reduced growth of M. brassicae but increased feeding by C. rubiginosa. Another species, Cladosporium cladosporioides, increased beetle feeding also, but had no effect on M. brassicae. Interactions were also seen between fungal species and dual infection with C. cladosporioides and Trichoderma viride greatly reduced beetle feeding. It is concluded that endophytes have significant effects on foliar feeding insects that differ with degree of specialism of the herbivore. We suggest that these effects are due to chemical changes in the host, brought about by fungal infection. These fungi have received remarkably little attention in the study of insect-plant interactions and yet could be important determinants of insect growth and even population dynamics.