Two xylariaceous fungi were isolated from a nest of a termite, Odontotermes formosanus, that was incubated in a laboratory after collecting from Iriomote Is., Okinawa Pref., in Japan. One of the two fungi was identified as Xylaria angulosa on the basis of the morphology of branched stroma produced on medium, tiny asci, and ascospores having a germ slit. Another fungus is an anamorphic fungus that produces synnemata up to 50 mm long from which dendritic conidiophores branch out. Unicellular conidia are holoblastically produced on a sympodially proliferating conidiogenous cell. Such morphological characters resemble those of the genus Geniculosporium. However, its distinctive synnema formation and dendritic conidiophores do not assign the fungus to Geniculosporium or other known genera and warrant establishment of a new genus. The phylogenetic tree based on the ITS regions of rDNA shows that the fungus is nested in the cluster of the genus Nemania (Xylariaceae), whose species have mainly Geniculosporium-like anamorphs. We describe here the present anamorphic fungus as Geniculisynnema termiticola gen. et sp. nov., and discuss its phylogenetic and ecological relationships to xylariaceous fungi, especially termiticolous species.