Perennial ryegrass (Lolium perenne) and tall fescue (Festuca arundinacea) plants were imparted the resistance to the bluegrass webworm (Parapediasia teterrella Zincken) by artificial infection with Acremonium sp. and A. coenophialum, respectively. The Acemonium endophytes were isolated from perennial ryegrass and tall fescue plants which were resistant to the larvae of the bluegrass webworm. The endophytes were inoculated by inserting the mycelia from the pure culture into the meristematic region. The presence of hyphae of the Acremonium endophytes in the inoculated plants was monitored by light microscopy 1-2 months after inoculation. The percentages of successful infection of perennial ryegrass with Acremonium sp. were 9-33%, and that of tall fescue with A. coenophialum was 7%. There were no differences in the appearance between the endophyte-infected plants and uninfected ones. The plants infected with the endophytes had acquired a feeding deterrent to the bluegrass webworm and the survival rate of the larvae on the infected leaf blades was definitely lower than that of the larvae on the uninfected leaf blades. The endophytes were transmitted to all seeds which were produced by the artificially infected perennial ryegrass and tall fescue plants. The plants grown from the seeds had also acquired the feeding deterrent to bluegrass webworm.