We describe the genetic analysis of 21 Escherichia coli strains in which the amino-terminal sequence of beta-galactosidase has been removed and replaced by an amino-terminal sequence from one or another of the proteins involved in maltose transport. Genetic mapping of the lacZ end of these fused genes indicates that only those fusions in which fewer than 41 amino acids are removed from the amino-terminal sequence of beta-galactosidase result in enzymatically active molecules. Within the region between amino acid 17 and amino acid 41 there are at least four or five sites where enzymatically active hybrid proteins can be formed.