The killing action and induced mutagenicity in hydroxylamine (HA)-treated cells of two moderately halophilic species of the genus Halomonas, H. elongata and H. meridiana, were investigated. A high sensitivity of H. elongata and especially of H. meridiana to HA was found. The efficiency of the mutagenicity obtained with the HA treatment was tested at different salinities. Optimal experimental conditions for HA mutagenesis of these two moderate halophiles were determined. A clear, efficient mutagenicity of both H. elongata and H. meridiana after HA mutagenesis was achieved. The optimal procedures yielded a number of useful auxotrophic mutants of H. elongata as well as different salt-sensitive mutants of H. elongata and H. meridiana for further studies. Some of these latter mutants appeared to be affected in the synthesis of compatible solutes.