For the genetic dissection of sexual sporulation in Aspergillus nidulans, we started a collection of ascosporeless mutants. After mutagenization of conidiospores with high doses of UV, we isolated 20 mutants with defects in ascospore formation. We crossed these mutants in two successive rounds with the wild-type strain. Eighteen of the 20 isolated mutants produced progeny with the original mutant phenotype in these crosses, and these mutants were further analyzed. All 18 analyzed mutations were recessive to wild type. We assigned them to 15 complementation groups, based on crosses between mutants. The mutants could be classified as follows according to their cytological phenotype: (1) no croziers, (2) arrest at prekaryogamy, (3) arrest in early meiotic prophase, (4) arrest in late meiotic prophase, (5) arrest in meiotic metaphase I, (6) defective postmeiotic mitosis and/or deliniation of ascospores, and (7) slow progression through the postmeiotic stages of ascospore formation. A large proportion of the mutants, namely 11 of 18, arrested in meiotic prophase or metaphase I. We discuss a possible approach for isolating the wild-type alleles of the genes that carry the sexual sporulation mutations.