Collections of black fly (Diptera: Simuliidae) larvae were made at previously unexplored lotic habitats in northern Thailand and their gut fungi identified. Eight trichomycete species were recovered, including one new species, Harpella asiana, and two possible new species: Graminelloides cf. biconica and Harpella cf. tica. Paramoebidium sp., Pennella montana Lichtwardt, Stachylina nana Lichtwardt and Stipella vigilans Léger & Gauthier are new Thailand records. The widely distributed species of gut fungus, Harpella melusinae Léger & Duboscq, that has previously been reported from Thailand, was also recovered. We have shown that trichomycetes are abundant in northern Thailand and undoubtedly many other Harpellales, beyond the taxa we report here, are present especially in unexplored regions of the country and in different hosts, habitats and times of the year. These findings correspond to 40 (6/15) of the trichomycete genera previously reported from tropical regions worldwide.