Mutants of E. coli defective in susceptibility to UV-induction of mutations were isolated by direct screening for their UV nonmutable phenotype (Umu-). Screening of about 30,000 mutagenized clones of a uvr-B derivative of AB1157 yielded six Umu- strains. The mutants can be classified into three groups by the location of the mutations, umuA, umuB and umuC. Mutations umuA and umuB are, respectively, mapped close to lexA and recA genes and mutations at both loci partially reduce UV mutagenesis. The locus of umuC is between hemA and purB and the mutations at this new locus result in a moderate increase of UV sensitivity. The mutation diminishes UV mutagenesis and UV reactivation of phage lambda without affecting the inducibility of phophage lambda nor the inhibition of cell division following UV irradiation. Related properties of an isogenic strain of a recF- mutant are compared with those of umuC-.