Plasmid pGC91. 14 harboring the transposon Tn951, which codifies for lac operon, was introduced by conjugation in Rhodobacter capsulatus. Nevertheless β-galactosidase activity in these strains was so low that they were unable to grow with lactose as the only carbon source. For this reason, chromosomal mutants of the R. capsulatus (pGC91. 14) strain able to grow on lactose minimal medium were isolated by mutagenesis with N-methyl-N′-nitro-N-nitrosoguanidine. In these mutants, the transcription of the lac operon was stimulated by the addition of lactose but not by the analogue isopropyl-β-d-thiogalactopyranoside. Furthermore, and contrary to Escherichia coli cells, addition of either cAMP or glucose to the cultures of the Lac+ mutants of R. capsulatus (pGC91. 14) did not produce any effect on the expression of β-galactosidase. © 1988 Springer-Verlag New York Inc.