Mutants with altered sexual reproduction were isolated from a homothallic wild-type strain ofNectria haematococca(Berk. & Br.) Wr. and genetically analysed. Most were self- and inter-sterile but some were interfertile and were used to improve genetic analysis by allowing an easy detection of hybrid perithecia. The study of these mutants also provides information about various aspects of the perithecial development in this organism.