Random Tn5 mutagenesis of the purple non-sulphur bacterium Rhodospirillum rubrum S1 allowed the isolation of a photosynthetically competent blue-green mutant, R. rubrum ST2. Southern hybridization showed that the Tn5 element was present as a single copy within the chromosome and distant from the puf operon. Phenotypic characterization of the mutant showed that the spectral characteristics of the photosynthetic complexes and water-soluble cytochromes were identical to those of the commonly used mutant R. rubrum G9. The protein SDS-PAGE profiles of the water-soluble and membrane fractions were essentially identical for both R. rubrum G9 and ST2. R. rubrum ST2 is suggested to be useful for structural and functional studies of bacterial photosynthesis.