Results from pot and microcosm studies in the greenhouse have shown that plant growth and foliar chemistry is altered by the presence and species composition of arbuscular mycorrhizal fungi (AMF). The growth and survival of herbivores which feed on plants could as a consequence also be affected by these mutualistic soil fungi. Consequently interactions between AMF plants and herbivores could occur. To test this larvae of the common blue butterfly Polyommatus icarus (Lycaenidae) were fed with sprigs of Lotus corniculatus (Fabaceae) plants which were inoculated with one of two different AMF species with a mixture of these AMF species or with sprigs of plants which were not inoculated with AMF Survival and larval weight of third instar larvae fed with plants colonised by AMF were greater than those of larvae fed with non-mycorrhizal plants. Survival of larvae feeding on non-mycorrhizal plants was 1.6 times lower than that of larvae feeding on plants inoculated with a mixture of AMF species and 3.8 times lower than that of larvae feeding on plants inoculated with single AMF species. Furthermore larvae fed with non-mycorrhizal plants attained only about half the weight of larvae fed with mycorrhizal plants after 11 days of growth. These differences in larval performance might be explained by differences in leaf chemistry since mycorrhizal plants had a 3 times higher leaf P concentration and a higher C/N-ratio. Our results thus show that the presence of belowground mutualistic soil fungi influences the performance of aboveground herbivores by altering their food quality. Larval consumption larval food use and adult lipid concentrations of the common blue butterfly differed between larvae which were fed with plants inoculated with different AMF species. This suggests that the performance of herbivores is not only influenced by the presence of AMF but also depends on the identity of the AMF species colonising the host plants. Moreover a significant interaction term between AMF species and maternal identity of the larvae occurred for adult dry weight indicating that the performance of offspring from different females was differently influenced by AMF species composition. To our knowledge these results show for the first time that the species composition of AMF communities can influence life-history traits of butterfly larvae and possibly herbivores in general.