To analyze the effect of cyclic AMP on the expression of the ompA gene of Escherichia coli, encoding the outer-membrane protein OmpA, a fusion between this gene and the lacZ gene was constructed in vitro by using a promoter-probe plasmid. The results obtained indicated that the presence of glucose in the culture medium decreased the transcription of the ompA gene. Likewise, cya and crp mutants exhibited lower levels of ompA gene expression than the wild-type strain. Furthermore, the addition of cyclic AMP increased the expression of the ompA gene in both cya and wild-type strains but not in a crp mutant. All these data show that the cyclic AMP receptor protein-cyclic AMP complex positively modulates ompA transcription in E. coli K-12.