Endophytes with intercellular mycelium characteristic of Acremonium coenophialum were isolated from tall fescue ( Festuca arundinacea) plants grown from 38 seed collections of cultivars and ecotypes from Europe, Algeria, North America, Australia and New Zealand. Thirty isolates were characterized by length of conidia, colony appearance and growth rate, sensitivity to benomyl, inhibition of other fungi in vitro, and effect on the feeding of Rhopalosiphum padi aphids on the host plants. Considerable cultural, morphological and physiological variation existed among the isolates examined. Most isolates resembled A. coenophialum morphologically, with conidia averaging 8·1–11·9 μm long, and protected their hosts from infestation by R. padi. Five isolates had smaller conidia than A. coenophialum, mean 5·5–6·7 μm long, and failed to deter aphids. The conidia of these isolates resembled A. typhinum and thus may be non-choke-inducing strains.