About 70 y after the first report by Yosio Kobayasi, we collected a little-known synnematous hyphomycete, Hymenostilbe mycetophila, on decaying basidiocarps of Favolaschia nipponica growing on dead culms of two bamboo species in Tateyama (on Sasa kurilensis), Chichibu (type locality of H. mycetophila; on Sasamorpha borealis), and Shigakogen (on S. kurilensis) in Japan, and obtained isolates of both the parasite and its host. Kobayasi provided only brief Latin and Japanese descriptions and quite elementary illustrations for H. mycetophila, without depositing any herbarium specimens including the holotype. In this paper, a lectotype and an epitype of this species are designated respectively from the original protolog (illustration) of Kobayasi and our new material collected in Chichibu. Phialidic conidiogenesis and 2- to 3-level verticillate conidiophores terminating in a whorl of 2-5 phialides were newly observed in this fungus on the natural substrate. ITS-LSU sequences of five isolates of H. mycetophila collected in the three locations were identical. Phylogenetic analyses of these markers placed H. mycetophila in Leotiales, although other species of Hymenostilbe are classified in Hypocreales at present. A new genus Kobayasiyomyces for H. mycetophila and a new combination, K. mycetophilus, are proposed here based on our morphological, ecological and phylogenetic data.