During sexual morphogenesis, the filamentous ascomycete Sordaria macrospora differentiates into multicellular fruiting bodies called perithecia. Previously it has been shown that this developmental process is under polygenic control. To further understand the molecular mechanisms involved in fruiting body formation, we generated the protoperithecia forming mutant pro1, in which the normal development of protoperithecia into perithecia has been disrupted. We succeeded in isolating a cosmid clone from an indexed cosmid library, which was able to complement the pro1(-) mutation. Deletion analysis, followed by DNA sequencing, subsequently demonstrated that fertility was restored to the pro1 mutant by an open reading frame encoding a 689-amino-acid polypeptide, which we named PRO1. A region from this polypeptide shares significant homology with the DNA-binding domains found in fungal C6 zinc finger transcription factors, such as the GAL4 protein from yeast. However, other typical regions of C6 zinc finger proteins, such as dimerization elements, are absent in PRO1. The involvement of the pro1(+) gene in fruiting body development was further confirmed by trying to complement the mutant phenotype with in vitro mutagenized and truncated versions of the pro1 open reading frame. Southern hybridization experiments also indicated that pro1(+) homologues are present in other sexually propagating filamentous ascomycetes.