BacA is an integral membrane protein, mutation of which leads to increased resistance to the antimicrobial peptides bleomycin and Bac71-35 and a greater sensitivity to SDS and vancomycin in Rhizobium leguminosarum bv. viciae, R. leguminosarum bv. phaseoli and Rhizobium etli. Growth of Rhizobium strains on dicarboxylates as a sole carbon source was impaired in bacA mutants but overcome by elevating the calcium level. While bacA mutants elicited indeterminate nodule formation on peas, which belong to the galegoid tribe of legumes, bacteria lysed after release from infection threads and mature bacteroids were not formed. Microarray analysis revealed almost no change in a bacA mutant of R. leguminosarum bv. viciae in free-living culture. In contrast 45 genes were more than 3-fold up regulated in a bacA mutant isolated from pea nodules. Almost half of these genes code for cell membrane components, suggesting that BacA is crucial to alterations that occur in the cell envelope during bacteroid development. In stark contrast, bacA mutants of R. leguminosarum bv. phaseoli and R. etli elicited formation of normal determinate nodules on their bean host, which belongs to the phaseoloid tribe of legumes. Bacteroids from these nodules were indistinguishable from wild type in morphology and nitrogen fixation. Thus while bacA mutants of bacteria that infect galegoid or phaseoloid legumes have similar phenotypes in free-living culture, BacA is only essential for bacteroid development in indeterminate galegoid nodules.