Salmonella plasmid virulence (spv) genes are organized into two transcriptional units: one formed by the spvR gene and the other by the spvA, spvB, spvC and spvD genes. Transcription of both units is activated by SpvR, a regulatory protein of the LysR family. The effect of RpoS, a stationary phase-associated sigma factor, on the expression of spv genes was studied using lacZ transcriptional fusions to spvR and spvA in wild-type and rpoS Escherichia coli backgrounds. Mutant and wild-type SpvR proteins were expressed in trans from a multicopy plasmid. The results show that the combined action of rpoS and spvR is necessary for transcription of spvA and that this combination also enhances transcription of spvR. Interestingly, spvR can also be transcribed in an alternative manner, i.e. in the absence of rpoS or spvR or both. The possible role for SpvR as a repressor of its own transcription is discussed.