UV sensitive mutants have been isolated and used extensively to gain insight into recombination mechanisms. Its effectiveness may be due to the close relationship of DNA repair and recombination. To search for the genes that may be involved in genetic recombination process, the frequency of meiotic or mitotic crossing over on the linkage group I was examined with several UV sensitive mutants in Aspergillus nidulaus. UV induced mitotic crossing over at quiscent or germinating cells was also examined. Most of the mutants showed normal meiotic or mitotic crossing over. However, some of them were altered in UV induced mitotic crossing over.