A world-wide study of the Oidium species causing economic damage on tomato has identified two taxa using classical morphological, scanning electron microscope (SEM) and molecular phylogenetic analyses. The material consisted of a total of 25 tomato powdery mildew isolates and 29 herbarium specimens coming from all continents where tomatoes are grown. A taxon with non-catenate conidia widespread in Europe, Africa, North and South America and Asia was identified as an O. subgen. Pseudoidium species (teleomorph: Erysiphe sect. Erysiphe). Formerly mistaken for O. lycopersicum (or O. lycopersici), it is now recognised as a distinct species, O. neolycopersici sp. nov. A phylogenetic analysis of the internal transcribed spacer (ITS) sequences of the ribosomal DNA (rDNA) indicated that O. neolycopersici is closely related to Erysiphe macleayae, E. aquilegiae and other Pseudoidium species. Only a taxon with catenate conidia was found on Australian specimens. This was identified as a species of O. subgen. Reticuloidium (teleomorph: Golovinomyces sp.). Phylogenetic analysis of the rDNA ITS sequences showed that this species is closely related to O. longipes infecting eggplant. Because it is most likely to be the same species as the original O. lycopersicum, which was actually first described in Australia, this is here neotypified as O. lycopersici.