The RsbT serine kinase has two known functions in the signal transduction pathway that activates the general stress factor sigmaB of Bacillus subtilis. First, RsbT can phosphorylate and inactivate its specific antagonist protein, RsbS. Second, upon phosphorylation of RsbS, RsbT is released to stimulate RsbU, a PP2C phosphatase, thereby initiating a signalling cascade that ultimately activates sigmaB. Here we describe a mutation that separates these two functions of RsbT. Although the mutant RsbT protein had essentially no kinase activity, it still retained the capacity to stimulate the RsbU phosphatase in vitro and to activate sigmaB when overexpressed in vivo. These results support the hypothesis that phosphatase activation is accomplished via a long-lived interaction between RsbT and RsbU. In contrast, RsbT kinase activity was found to be integral for the transmission of external stimuli to sigmaB. Thus, one route by which environmental stress signals could enter the sigmaB network is by modulation of the RsbT kinase activity, thereby controlling the magnitude of the partner switch between the RsbS-RsbT complex and the RsbT-RsbU complex.