Achnatherum inebrians (drunken horse grass), a grass that is intoxicating to grazing animals and commonly found within native grasslands of China, is host to a symbiotic endophyte, Neotyphodium gansuense. In laboratory tests, N. gansuense infections of A. inebrians increased mortality rates and reduced the survival of an aphid Rhopalosiphum padi and a mite Tetranychus cinnabarinus. Both species demonstrated a quantitative preference for feeding on endophyte-free (E)) compared with endophyte-infected (E+) leaves. In the field, populations of the two species were lower on E+ compared with E) specimens for most of the growing season. Results of tests with a grasshopper species (Oedaleus decorus) gave similar results. A species of seed-harvesting ant (Messor aciculatus) showed a preference for collecting E) seeds. These results demonstrate that the antiherbivore activity of E+ A. inebrians is effective against a wide range of arthropod groups. Thus, the Neotyphodium endophyte may help this grass to improve its invasive ability and, therefore, broaden its range.