A morphologically interesting hyphomycete was collected from submerged wood in a stream in Doi Suthep-Pui National Park, Thailand. It is described as Aquaticheirospora lignicola gen. and sp. nov.,and is characterized by euseptate conidia with divergent arms, which are vertically inserted in different planes to a basal cell. The genus differs from other chirosporous genera in having synnematous conidioma and conidia that are produced on conidiogenous cells borne at the apices of synnemata. The morphological characterization of this new fungus is reported and compared with similar chirosporous genera. To investigate the teleomorphic and phylogenetic relationships of this new taxon, three different regions of the ribosomal gene [18S rDNA, 28S rDNA, and internal transcribed spacer (ITS) including 5.8S] were sequenced and analysed. The results of phylogenetic analyses based on 18S, 28S, and partial ITS including 5.8S rDNA, employing different tree-making methods, indicate that Aquaticheirospora lignicola is closely related to the ascomycetes family Massarinaceae (Order: Pleosporales). The Massarinaceae as currently circumscribed is monophyletic. Massarina australiensis and M. bipolaris, however, appear to belong to the Lophiostomataceae. © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007, 155, 283–296.