The non-ostiolate character of the perithecia of Microthecium is used to separate it as a genus distinct from Melanospora which is ostiolate. Species and varieties accepted in Microthecium are M. zobelii Corda, the type, M. levitum sp. nov., M. epimyces, M. geoporae, M. hypomyces, M. compressum sp. nov., M. retisporum sp. nov., and M. retisporum var. inferior var. nov. The generic description is emended. A key to the species is presented. Six species previously included in Microthecium are excluded. Melanospora tulasnei sp. nov. is described.