Different levels of β-galactosidase are found in various trp-lac fusion strains. These levels of β-galactosidase fall within a 60-fold range. The amount of thiogalactoside transacetylase activity detected in these same strains only varies 10-fold and is found in amounts greater than those predicted from the β-galactosidase levels. The observation that the β-galactosidase and thiogalactoside transacetylase levels are not directly proportional, that the lacZ messenger ribonucleic acid (mRNA) levels are not proportional to the β-galactosidase activity, that, at least for the one fusion strain tested, the SuA polarity suppressor does not affect the β-galactosidase level, and that, in all but one strain, the β-galactosidase activity appears to reside in normal β-galactosidase molecules suggests that the disproportionately low production of β-galactosidase is due to a decrease in the frequency of translation initiation of lacZ mRNA in these strains. Several mechanisms are proposed to explain this decrease. Some possible bases for the disproportional production of β-galactosidase and thiogalactoside transacetylase are also described. The preferred explanation for these disproportional enzyme levels is that only a fraction of the full complement of ribosomes need initiate translation at lacZ for the functional synthesis of lac mRNA to occur and that once the lac ribonucleic acid is made a full complement of ribosomes can bind at internal translation initiation sites at Y and A.