Bordetella pertussis and Bordetella bronchiseptica contain nearly identical BvgAS signal-transduction systems that mediate a biphasic transition between virulent (Bvg+) and avirulent (Bvg-) phases. In the Bvg+ phase, the two species express a similar set of adhesins and toxins, and in both organisms the transition to the Bvg- phase occurs in response to the same environmental signals (low temperature or the presence of nicotinic acid or sulphate anion). These two species differ, however, with regard to Bvg(-)-phase phenotypes, host specificity, the severity and course of the diseases they cause, and also potentially in their routes of transmission. To investigate the contribution of the virulence-control system to these phenotypic differences, we constructed a chimeric B. bronchiseptica strain containing bvgAS from B. pertussis and compared it with wild-type B. bronchiseptica in vitro and in vivo. The chimeric strain was indistinguishable from the wild type in its ability to express Bvg(+)- and Bvg(-)- phase-specific factors. However, although the chimeric strain responded to the same signals as the wild type, it differed dramatically in sensitivity to these signals; significantly more nicotinic acid or MgSO4 was required to modulate the chimeric strain compared with the wild-type strain. Despite this difference in signal sensitivity, the chimeric strain was indistinguishable from the wild type in its ability to cause respiratory-tract infections in rats, indicating that the bvgAS loci of B. pertussis and B. bronchiseptica are functionally interchangeable in vivo. By exchanging discrete fragments of bvgAS, we found that the periplasmic region of BvgS determines signal sensitivity.