More than 200 mutants of Aspergillus nidulans were isolated as hypersensitive to the monofunctional alkylating agent MNNG and/or UV-irradiation (designated nuv mutants). Of these, 23 were selected for further characterization. All were markedly hypersensitive to both MNNG and the quasi-UV-mimetic mutagen 4-NQO. The hypersensitive phenotype of each mutant was shown to result from mutation of a single gene. The nuv mutants exhibited a diverse range of growth responses on solid media containing various concentrations of MNNG or 4-NQO. This suggested that they represented many nonallelic mutations. Analysis to determine the dominance/recessiveness of the nuv mutations with respect to hypersensitivity revealed that most were fully recessive, although several appeared to be semidominant. A novel system to assay homologous mitotic recombination using simple plating tests was developed. The system was exploited to determine the effects of the nuv mutations on mitotic recombination. Of the 23 mutations tested, 10 caused a hypo-recombination phenotype and three a hyper-recombination phenotype, while 10 appeared to have no effect on recombination. The hypo-rec effect of one of the mutations, nuv-117, appeared to be semidominant. Transcomplementation analysis between seven of the nuv mutations defined at least six nonallelic loci.