Technical Abstract: The fungi of genus Neotyphodium are systemic, constitutive, symbionts of grasses of subfamily Pooideae. In the southern hemisphere most of these asexual endophytes are the result of the hybridization between two sexual species, Epichloe festucae and E. typhina, from the northern hemisphere. However the ancestral sexual species have not been detected in this region. Several grasses from Argentina are infected by Neotyphodium species. These endophytes are in general very similar macro-and micromorphologically and phylogenetically conform to species N. tembladerae. However the Neotyphodium spp. endophytes of some hosts, Bromus auleticus and Poa spicifomis var. spiciformis, have not been included in this species. In this work we studied the incidence and characterized the diversity of Neotyphodium species in populations of the native grass Bromus auleticus from Argentina. The incidence of endophytes was 100% in all populations investigated. Two groups of endophytes were differentiated by their morphologies, growth rates, conidial ontogenies and by relative resistance to the fungicide benomyl. Phylogenetic trees inferred from tefA and tubB intron sequences indicated that both N. tembladerae and the novel morphotype were hybrids of E. festucae and E. typhina, but the ancestral E. typhina genotype distinguished them. Isolates from plants that inhabit coastal dunes, xerophytic forests, savannahs and hills were similar morphologically and phylogenetically to N. tembladerae, whereas the endophytes from the humid pampa plains conformed to the novel group. We propose the endophyte of Bromus auleticus from humid pampas as a new species, Neotyphodium pampeanum.