The order Tilletiales (Ustilaginomycetes, Basidiomycota) includes six genera (Conidiosporomyces, Erratomyces, Ingoldiomyces, Neovossia, Oberwinkleria and Tilletia) and approximately 150 species. All members of Tilletiales infect hosts in the grass family Poaceae with the exception of Erratomyces spp., which occur on hosts in the Fabaceae. Morphological features including teliospore ornamentation, number and nuclear condition of primary basidiospores and ability of primary basidiospores to conjugate and form an infective dikaryon were studied in conjunction with sequence analysis of the large subunit nuclear rDNA gene (nLSU). Analysis based on nLSU data shows that taxa infecting hosts in the grass subfamily Pooideae form one well supported lineage. This lineage comprises most of the reticulate-spored species that germinate to form a small number of rapidly conjugating basidiospores and includes the type species Tilletia tritici. Two tuberculate-spored species with a large number of nonconjugating basidiospores, T. indica and T. walkeri, and Ingoldiomyces hyalosporus are also included in this lineage. Most of the species included in the analysis with echinulate, verrucose or tuberculate teliospores that germinate to form a large number (>30) of nonconjugating basidiospores infect hosts in the subfamilies Panicoideae, Chloridoideae, Arundinoideae and Ehrhartoideae. This group of species is more diverse than the pooid-infecting taxa and in general do not form well supported clades corresponding to host subfamily. The results of this work suggest that morphological characters used to segregate Neovossia, Conidiosporomyces and Ingoldiomyces from Tilletia are not useful generic level characters and that all included species can be accommodated in the genus Tilletia.