The regulatory region of the ompA gene from Escherichia coli has been characterized by biochemical and genetic approaches. Two overlapping promoters, P1 and P2, organized in that order with respect to the ompA coding sequence, were identified and it was found that ompA possesses an unusually long leader region. Both P1 and P2 were active in an in vitro transcription system although S1 mapping analysis of the ompA mRNA made in vivo showed that P2 was mainly responsible for transcription of the gene. Confirmation of this was obtained by studying down-promoter mutants of ompA cloned in pSC101. These mutants were classified into two groups, deletions and insertions. The deletions, which were caused by the IS102 insertion element found in pSC101 removed the--35 regions of both P1 and P2. However, since P2 was distally situated with respect to the IS element it was less extensively damaged and it is proposed that the residual P2 sequence is responsible for the low level of expression observed. In addition to an IS102 insertion in the promoter region four IS1 insertion mutants were characterized. These had integrated at different positions in the ompA leader region and were all incompletely polar.