We have isolated a series of strains in which the lacZ gene has been fused to one of the maltose operons, such that the synthesis of beta-galactosidase (beta-D-galactoside galactohydrolase; EC 3.2.1.23) is inducible by maltose. The most frequent event that generates such fusions results in strains in which an intact lacZ gene has become a part of the malE,F operon. By using a special selection procedure, we have detected much rarer fusion events resulting in an altered beta-galactosidase molecule. In these strains, we presume that there is a hybrid protein molecule produced, comprised of an NH2-terminal amino acid sequence from a maltose transport protein (malF) and a COOH-terminal amino acid sequence from beta-galactosidase. The hybrid protein, which still retains some beta-galactosidase activity, is found in the cytoplasmic membrane. These results provide information on the component of the malF gene essential for incorporation of its product into the membrane.