The 5' untranslated region (UTR) of the long-lived Escherichia coli ompA message can function in vivo as an mRNA stabilizer. Substitution of this ompA mRNA segment for the corresponding segment of the labile bla gene transcripts prolongs their lifetime by a factor of 6. We show here that the function of this ompA mRNA stabilizer requires the presence of a 115-nucleotide ompA RNA segment that lies upstream of the ribosome-binding site. Although deletion of this segment reduced the half-life of the ompA transcript by a factor of 5, its absence had almost no effect on the translational efficiency of ompA mRNA. Like the ompA transcript, but unlike bla mRNA, hybrid ompA-bla messages containing the complete ompA 5' UTR were significantly less stable under conditions of slow bacterial growth. We conclude that the stabilizing activity of the ompA 5' UTR is growth rate regulated and that the mechanism of mRNA stabilization by this RNA segment is not related to the spacing between translating ribosomes.