The isolation from strains of Micrococcus lysodeikticus of nucleases capable of removing thymine dimers from ultraviolet (UV) irradiated DNA (Carrier and Setlow, 1966; Takagi et al., 1968; Grossman et al., 1968) has prompted the isolation of UV sensitive mutants of M. lysodeikticus. This paper describes the properties of three selected mitomycin sensitive mutants, which are UV-. It also describes the transformation of these mutants with wild type DNA and the characteristics of the transformants obtained. © 1968.