Poor survival and reproduction of the aphidRhopalosiphum padi(L.) was observed on tall fescue plants infected with the endophytic fungusAcremonium coenophialumMorgan-Jones&Gams. Reproduction was always much higher on uninfected plants, but on older infected plants, someR. padisurvived long enough to molt and reproduce. In choice studies,R. padiand the greenbug,Schizaphis graminum(Rondani), preferred uninfected fescue plants. Greenbugs quickly destroyed their uninfected hosts and died. ThoseS. graminumthat moved to infected plants also died. High numbers ofR. padion fescue plants could be a reliable indicator for the absence of endophyte-associated alkaloids that cause resistance to a number of other insect herbivores.