Although the genes of the aceBAK operon are expressed from the same promoter, the relative cellular levels of their products are approximately 0.3:1:0.003. Gene and operon fusions with lacZ were constructed to characterize this differential expression. The upshift in expression between aceB and aceA resulted from differences in translational efficiency. In contrast, inefficient translation and premature transcriptional termination contributed to the downshift in expression between aceA and aceK. Premature transcriptional termination occurred within aceK and appears to result from inefficient translation. Deletion of repetitive extragenic palindromic elements between aceA and aceK had little effect on the relative expression of these genes. Rather, the sequences responsible for inefficient expression of aceK lie within the aceK ribosome binding site.