Neotyphodium coenophialum is a fungus that commonly grows endosymbiotically within cool season grasses. The fungus can provide protection against biotic stresses to its host through the production of alkaloids. We tested if combinations of two cultivars (Georgia 5 and Jesup) of tall fescue with different isolates of N. coenophialum alter resistance to the aphid, Rhopalosiphum padi, in a greenhouse experiment. Aphid populations grew faster on uninfected plants of both cultivars. In Georgia 5, all fungal isolates depressed aphid reproduction, whereas only one of three did so in Jesup. In choice experiments, we found aphids preferred uninfected blades over blades infected with any of the fungal isolates. We conclude that R. padi is sensitive to a variety of fungal isolate and plant cultivar combinations, but that genetic variation in both plant and fungus can influence the degree of resistance provided by the fungus.