A new parasitic fungus, Laboulbenia curtonoti sp. n., associated with the endemic Madagascan fly Curtonotum balachowskyi Tsacas (Diptera: Curtonotidae), is described and figured. The new species is one of the very few Laboulbeniales which penetrate the insect's cuticle by means of more-or-less developed rhizoids. A brief review of the 23 species of the genus Laboulbenia associated with Diptera is presented in a tabulated form.