Summary An unknown fungus, Stagonospora sp., has been found on and in stromata of Botryosphaeria stevensii Shoemaker anamorph (Diplodia mutila Fr. & Mont.) and Diatrypella quercina (Persoon) Nitschke. Microscopic investigations indicated that the fungus might be a hyperparasite of some ascomycetous fungi, necrotrophs or weak parasites on sessile oak [Quercus petraea] and turkey oak (Quercus cerris). Dual culture studies carried out with monoconidial isolates of Stagonospora sp. and B. stevensii have demonstrated that Stagonospora sp. is a necrotrophic mycoparasite which might suppress, to some extent, the natural population of B. stevensii. Botryosphaeria stevensii is one of the biotic factors causing oak decline. Morphology of the fungus and symptoms of mycoparasitic interaction are described. Stagonospora sp. found in Hungary is assumed to be identical with hyperparasitic Stagonospora sp. reported from Germany and Austria as parasite of Ascodichaena rugosa and Ascodichaena mexicana in Mexico. This is the first record of hyperparasitic Stagonospora sp. of B. stevensii and D. quercina.