An UV sensitive mutant, B5, isolated from Asp. nidulans, was extremely sensitive to UV but slightly to monofunctional alkylating agent. UV sensitivity increases up to conidia germination. The UV irradiated cells were photoreactivated to almost same level as wild type. Liquid holding recovery did not affect survival rate of the UV irradiated cells and liquid holding recovery after the first irradiation did not increase the cell survival rate by secondary UV irradiation. The mutation produced the unusually high frequency of aneuploid formation. The mutant was deficient in UV induced mutagenesis. No ascospore was developed in homozygotic cross. Neither the diploid homozygous for mutant gene nor that heterozygous for mutant gene and uvsH1 was effectively crossed.