The nucleotide sequences of the intercistronic regions from several multicistronic operons have recently been determined. In many cases such intercistronic regions consist of only a few base pairs (bp); in certain extreme cases, where the initiation codon of one gene overlaps the termination codon of the previous gene, intercistronic regions are essentially absent1,2. However, in other cases intercistronic regions can be up to several hundred base pairs long; it seems reasonable to suppose that, at least in these cases, such regions have a role in regulating expression of the operon. Indeed, a regulatory function for the large intercistronic region of the rplJL-rpoBC operon has been demonstrated3. We have now identified a genetic element, common to intercistronic regions from three independent bacterial operons (the histidine transport and the histidine biosynthetic operons of Salmonella typhimurium and the malK-lamB operon of the malB region of Escherichia coli), which we believe to be important in regulating the differential expression of individual genes in these multicistronic operons. A similar element is also found immediately following the trpR gene of E. coli although in this case it is not known whether or not a distal gene exists. Each of these elements consists of a long dyad symmetry which, once transcribed, could form an exceptionally stable stem-loop structure with a stem of up to 35 bp. In each case smaller palindromic units are also found, some of which overlap the main symmetry. The stem-loop structures show remarkable homology (---90%) and probably have a common evolutionary origin, possibly an insertion element. It seems that the function of these structures is to effect a decrease in expression of distal genes in an operon.