Transcriptional regulation of the rhaT gene, one of the operons forming the rhamnose regulon in Escherichia coli, was studied by fusing its complete or deleted promoter to the reporter gene lacZ. Analysis of p-galactosidase activities induced in these constructions grown under different conditions predicted the presence of two putative control elements: one for the RhaS regulatory protein and activating the gene not only by L-rhamnose but also by L-lyxose or L-mannose, the other for CAMP-catabolite repression protein and activating this gene in the absence of glucose. Anaerobiosis increased the promoter function two-to threefold with respect to the aerobic condition. Experiments involving complementation of strains containing the rhaTpromoter fusion and carrying a deletion in the rhaS and/or rhaR genes with plasmids bearing the rhamnose regulatory genes showed that rhaT is controlled by a regulatory cascade, in which RhaR induces rhaSR and the accumulated RhaS directly activates rhaT.