Nine mutagen sensitive (mus) mutants of Aspergillus nidulans were analyzed for effects on recombination. These mus show increased sensitivity to various mutagens; several were UV-sensitive. The majority were sterile in homozygous crosses homozygous mus diploids were essential to test for mitotic recombination. / Two distinct tests were used: intergenic recombination between colour mutations and their centromeres; and, intragenic recombination between two distinguishable adE alleles. Both hyper- and hypo-rec mus mutations were identified, with 15- to 20-fold differences between them. Three mutations musN, musO, and musQ had greatly elevated recombination frequencies. Only one, musL significantly reduced recombination but consequently increased malsegregation. / Analysis of the genotype of ad$ sp{+}$ recombinants, from musN and musL diploids, revealed qualitative differences from mus$ sp{+}$. musN enhanced mitotic crossing over but not gene conversion; while musL reduced the frequency of simple convertants but increased that of complex homozygous ad$ sp{+}$ genotypes. Mitotic conversion and crossing over, generally associated with each other, were uniquely affected in such mus diploids.

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