Abstract
A survey of freshwater ascomycetes conducted in Thailand yielded a number of aquatic hyphomycetes. In this study, fresh collections of three chaetosphaeriaceous species from submerged wood in freshwater are characterized based on morphology and molecular phylogeny. Dictyochaeta siamensis sp. nov. and Tainosphaeria siamensis sp. nov. are introduced based on morphological and molecular data. A detailed description of Menisporopsis theobromae from the new collection is presented and the first molecular data for this genus are provided. Phylogenetic analysis of combined LSU and ITS sequence data was carried out to determine the phylogenetic placement of these species within the family Chaetosphaeriaceae. Menisporopsis theobromae showed a close phylogenetic relationship with Rattania, but has significantly different morphology; D. siamensis clustered together with Codinaeopsis, Menispora, and ZignoA << lla; while T. siamensis clustered together with T. crassiparies but presented as a distinct clade. The phylogenetic relationship between the new taxa and their relatives are compared and discussed.