A new synnematous hyphomycete, Siamia luxuriosa gen. et sp. nov., is descri
bed from Khao Yai National Park, Thailand. The genus is characterised by ha
ving long, dark synnemata, conidiogenous cells with numerous protuberant, c
ylindrical, thickened unilaterally arranged scars, and long, slightly curve
d, rostrate, multiseptate conidia. Siamia can be compared with several gene
ra of synnematous hyphomycetes e.g. Phaeoisariopsis, Isariella, Atractilini
a or Cercospora.