Haptospora is described as a new genus of the Hyphomycetes endoparasitic in rotifers. The genus is distinguished by conidia that have a doughnut-shaped modification at the base. This structure appears to have a function in lodging the conidium in the mastax of the host. The conidiogenous cells are phialides, each with a membranous collarette. Two species of Hyphomycetes, described previously as Phialophora endoparasitica and P. tribrachispora, are transferred to Haptospora, and a third species, Haptospora appendiculata, is described as new. Key words: Phialophora, Haptospora, endoparasite, rotifer.