Three coelomycetous ascomycetes, Acrocalymma aquatic sp. nov., Aquasubmersa mircensis gen. & sp. nov., and Clohesyomyces aquaticus are reported from northern Thailand. All taxa are characterized, illustrated and subjected to awnalyses of LSU, SSU and/or ITS rDNA gene sequence data to establish their phylogenetic placement in the ascomycetes. Acrocalymma is a monotypic genus in a distinct lineage of Pleosporales which is basal to Morosphaeriaceae as confirmed by our combined SSU and LSU analysis. Acrocalymma aquatica sp. nov., differs in morphology and gene sequence data from other species in the genus. Aquasubmersa mircensis gen. et sp. nov. is phylogenetically unrelated to any other dothideomycetous genus and is introduced as a new genus. Sequence data from Clohesyomyces aquaticus facilitated its accommodation in Lindgomycetaceae.