The mutagenic and lethal effects of N-methyl-N'-nitro-N-nitrosoguanidine (NTG), ethyl methanesulphonate (EMS), ultraviolet light irradiation and near-ultraviolet light irradiation with 8-methoxypsoralen on the bacterium Acinetobacter calcoaceticus NCIB8250 were examined. The production of auxotrophic mutants was used as a measure of mutagenic efficiency. Under appropriate conditions all four agents were mutagenic. EMS and NTG although more effective than irradiation, did not cause such a high frequency of mutation as has been observed with other bacteria. A combination of vancomycin and penicillin V gave enrichment of non-metabolizing bacteria and optimum conditions were found for the use of these compounds in a selection technique.