The Pm promoter of the TOL plasmid of Pseudomonas putida is expressed at high level along the growth curve. This transcription is dependent on the positive regulator XylS activated by 3-methylbenzoate. The sigma factor σ38 is required for expression in early stationary phase and thereafter. To test whether σ70 was involved in Pm transcription in exponential phase, we have followed mRNA synthesis in a rpoD thermosensitive strain. No difference in Pm transcription was found between the wild type and the thermosensitive strain at the restrictive temperature of 42°C, indicating that transcription was independent of the sigma factor σ70. However, basal levels of mRNA expression from Pm in this strain in exponential phase were more than twofold higher at 42°C, suggesting involvement of σ32 in Pm transcription. In a rpoH background, no expression of Pm took place in the exponential phase, whereas it increased during stationary phase, and in a rpoH rpoS double mutant no activity from the Pm promoter was detected along the growth curve. We have shown that the increase in the amount of σ32 factor necessary for transcription in exponential phase is provided through induction of the heat shock response by the presence of the effector 3-methylbenzoate, which is also required for activation of the positive regulator XylS. We conclude that activation of Pm transcription is achieved through a switch between two stress-responsive factors, σ32 in exponential phase and σ38 in stationary phase. In both cases, transcription is dependent on the activator XylS and presents the same transcription start point.