The expression of the linked toxA and toxB genes coding for the A and B subunits of the heat-labile enterotoxin of Escherichia coli, respectively, has been studied. For this reason, toxA-lacZ and toxB-lacZ fusions were constructed through a promoter-probe vector. Results obtained show that the toxB gene has a promoter from which it may be transcribed independently from the toxA gene. Nevertheless, the expression of toxB gene is about 25-fold higher from the toxA promoter than from its own promoter. Furthermore, the presence of glucose in the culture medium decreased the transcription from both toxA and toxB promoters. Likewise, cya and crp mutants exhibited a lower level of tox gene expression than did the wild-type strain. The addition of cyclic AMP increased the expression from toxA and toxB promoters in both cya and wild-type strains, but not in a crp mutant. All these data suggest that the cyclic AMP receptor protein-cyclic AMP complex positively modulates toxA and toxB gene transcription in E. coli. © 1990 Springer-Verlag New York Inc.