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Acremonium is generally considered to be a highly polyphyletic form genus containing distantly related fungi. Sectional divisions within Acremonium distinguish the clavicipitaceous grass endophytes of sect. Albolanosa from the generally saprobic species of sections Acremonium, Chaetomioides, Gliomastix, and Nectrioidea. In an effort to assess the possible number of lineages currently placed within Acremonium and to determine which groups of sexual ascomycetes are phylogenetically affiliated with Acremonium species, maximum parsimony and neighbor-joining analyses were performed using partial sequences of the nuclear small subunit ribosomal DNA (18S rDNA). Acremonium was shown to be a polyphyletic taxon with affiliations to at least three ascomycetous orders: 1) most of the examined species from the sections Acremonium, Gliomastix, and Nectrioidea showed a relationship to the Hypocreaceae even though many of these species have never been associated with any teleomorph; 2) the grass endophytes of sect. Albolanosa and other taxa from the Clavicipitaceae formed a monophyletic group derived from within the Hypocreales; 3) the thermophilic A. alabamense of sect. Chaetomioides was derived from within the Sordariales. Acremonium alternatum, the type species of the genus, was one of the species showing affiliation to the Hypocreaceae. In order to eliminate some of the heterogeneity within Acremonium while also emphasizing the unique biological, morphological, and ecological characteristics of the grass endophytes, we are proposing that the anamorphs of Epichloë and closely related asexual grass endophytes be reclassified into the new form genus Neotyphodium. Phylogenetic and taxonomic considerations are also presented for other taxa.