Abstract: A new synnematous hyphomycete, Mycosylva reticulata Samson & Hintikka sp. nov. is described. It has been isolated from pellets of small rodents in northern Finland and differs from the type culture of M. clarkii Tulloch mainly by the reticulate ornamentations of the conidia, the presence of ramoconidia and the olive green colour of the synnematal heads. Mycosylva clarkii and M. reticulata are psychrophilic, showing optimal growth at l5 °C. The relationship of the genus Alycosylva to Myceliophthora, Hormoconis, Pycnostysanus and Heydenia is discussed.