We investigated whether there is translational coupling between the tryptophan operon trpB and trpA genes in Escherichia coli. A trp-lac fusion system was used in which part of the trpA gene is fused to the lacZ gene. This fusion protein has the translation initiation site of trpA but retains beta-galactosidase activity. We introduced a frameshift mutation early in trpB and measured its effect on transcription and translation of the trp-lac fusion. The mutation resulted in a 10-fold drop in beta-galactosidase activity but only a 2-fold drop in lacZ mRNA or galactoside transacetylase levels. An rho mutation restored the lacZ mRNA and transacetylase levels to those of the control but only increased the beta-galactosidase level to 20% that of the control. We conclude from these results that if the trpB gene is not translated, efficient translation of the trpA'-lac'Z mRNA does not occur and, thus, that these genes are translationally coupled. The implication of this finding for other studies with gene fusions is discussed.