Activity of a Na+/H+ antiporter has been suggested to be critically involved in pH homeostasis in obligately alkalophilic bacteria (Krulwich, K. A., Mandel, K. G., Bornstein, R. F., and Guffanti, A. A. (1979) Biochem. Biophys. Res. Commun. 91, 58-62) and in Escherichia coli (Zilberstein, D., Padan, E., and Schuldiner, S. (1980) FEBS Lett. 116, 177-180). A concern with respect to these proposals has been the failure of either Bacillus alcalophilus or E. coli to exhibit a requirement for added Na+ for growth. Thus, it became of interest to examine Na+-coupled porter functions in obligately alkalophilic Bacillus firmus RAB, a species that exhibits an absolute requirement for added Na+ for growth at pH 10.5. In a comparative study using membrane vesicles from B. alcalophilus and B. firmus RAB it was found that both the Na+/H+ antiporter and the Na+/alpha-aminoisobutyric acid symporter from the "Na+-requiring" species had much lower apparent affinities for Na+ than corresponding porters from B. alcalophilus. At high concentrations of Na+, the porters from the two species were functionally similar. These findings support the argument that the absence of a growth requirement for added Na+ may reflect an ability of at least some bacteria to effectively utilize and recycle the available levels of Na+ that contaminate all media, rather than reflect true Na+ independence. Studies with a nonalkalophilic derivative of B. firmus RAB confirmed earlier findings with B. alcalophilus of a pleiotropic loss of Na+ coupling to porters in nonalkalophilic mutants.