Tall fescue [Festuca arundinacea Schreb. var. arundinacea Schreb. (2n = 6x = 42)] breeding objectives are tn exploit the natural variation of the associated endophytic fungi and to select specific plant-fungus combinations that optimize the host fitness but do not cause detrimental effects on grazing animals. This study investigated the presence of endophytes in 60 tall fescue natural populations from Sardinia, Italy; identified the endophytes and assessed some important metabolites produced by their interaction with the host plant; and characterized the tall fescue populations for morphological traits, relating the variation among populations tn possible differences in the associated endophytes. The high frequency of infected populations (58 out of 60), and high levels of infection (on average, 80% infected seed), suggested an adaptive advantage of E + plants under harsh Mediterranean conditions. Morphological identification of fungal isolates, in comparison with Neotyphodium coenophialum (Morgan-Jones et Gams) Glenn, Bacon et Hanlin check isolates, made it possible to separate two groups of populations. One, infrequent, associated with a long-conidia endophyte (attributable to N. coenophialum), and another associated with a short-conidia form, likely belonging to the Festuca arundinacea Taxonomic Grouping-2 (FaTG-2), which was previously isolated in a few Mediterranean accessions. Chemical analysis revealed the presence of Inline alkaloids only in populations associated to the long-conidia endophyte, thus corroborating that form's attribution to the loline-inducing N. coenophialum. A difference was also observed in ergovaline concentration between long-conidia (= N. coenophialum) and short-conidia endophyte variants, the latter producing only about 25% of the ergovaline produced by the former. A coevolutionary specificity between the native Sardinian rescue germplasm and its associated endophyte was suggested by the agreement between morphology of the host plant (distinct from germplasm originating in temperate environments) and morphological and biochemical characteristics of the harbored fungus. Abbreviations: E+, endophyte-infected germplasm; E-, endophytefree germplasm.