Seed transmitted Neotyphodium endophytes are widespread in grasses. These fungi may modify agronomic plant traits and induce synthesis of mycotoxins that lead to possible cattle toxicoses. For these reasons, the Technical Permanent Committee for Breeding recommends the grass varieties submitted to the French Official List to contain less than 20% of seeds with living Neotyphodium. Several fungicides were tested as seed treatments on perennial ryegrass (Lolium perenne L.) and tall fescue (Festuca arundinacea Schreber). Phytotoxic effects were measured. Their efficacy was evaluated in greenhouse and in field conditions of seed production. Triticonazole [(RS)-(E)-5-(4-chlorobenzylidene)-2,2-dimethyl-l-(1H-1,2,4-triazol-1-ylmethyl)cyclo- pentanol] was effective but much too phytotoxic. The tested doses of bitertanol [1-(biphenyl-4-yloxy)-3,3-dimethyl-1-(IH-1,2,4-triazol-1-yl)butan-2-ol] and fluquinconazole [3-(2,"chlorophenyl)-6-fluoro-2-(1H-1,2,4-triazol-1-yl)quinazolin-4(3H) -one] have few negative effects on germination but very few effects on endophytes. Prochloraz [N-propyl-N-[2-(2,4,6-trichlorophenoxy)ethyl]-1H-imidazole-1-carboxamide] (0.75 g kg(-1)) represents the best balance between high efficacy and low phytotoxicity on perennial ryegrass and tall fescue seeds. No difference among varieties and species was detected in their response to prochloraz treatment. In greenhouse trials, less than 10% of living endophyte remained in perennial ryegrass and tall fescue seed lots, even in highly infected ones (66% of E+ seeds), when treated with prochloraz. This seed treatment was also efficient in field conditions where the level of endophyte was reduced by more than 60% in highly endophyte-infected ryegrass seed lots. This study shows that, even if seed germination is a little decreased, prochloraz could be used during grass breeding so that subsequent generations of seed will be free from endophyte with good germination rate.