The sulA gene of Escherichia coli, which encodes an inducible inhibitor of cell division, plays a role in the SOS response. Its expression, like that of other SOS genes, is repressed by the LexA protein. This paper reports the identification and characterisation of the promoter for the cloned sulA gene. The promoter bears good resemblance to the consensus promoter sequence and directs the synthesis of two major RNA species both in vitro and in vivo. Comparison of the sequence around the sulA promoter with the operator sequences of other SOS genes revealed the presence of an SOS box, the LexA protein binding site. This overlaps the -10 region of the promoter and covers the transcriptional initiation sites. LexA protein bound to this SOS box, would, therefore, effectively block transcription.