The active transport of maltose in Escherichia coli requires the products of five genes. These include a water-soluble periplasmic maltose-binding protein, three cytoplasmic membrane proteins, and an outer membrane protein. In order to evaluate the role of the maltose-binding protein in active transport, a nonpolar internal deletion of the structural gene for the maltose-binding protein was constructed. A strain which contains this deletion is unable to grow on maltose at an external concentration of 25 mM, even when the remaining components of the transport system are synthesized constitutively. This demonstrates that the periplasmic maltose-binding protein is essential for detectable translocation of maltose across the cytoplasmic membrane. Mal+ revertants of the deletion strain were obtained. In one of these strains, the remaining components of the maltose transport system gained the ability to translocate maltose across the membrane independently of the periplasmic binding protein. Maltose transport in this revertant strain is specific for maltose; it is not inhibited by other alpha and beta glucosides and galactosides. In contrast to the wild type, transport activity in the Mal+ revertant strain is retained by spheroplasts. The cytoplasmic membrane components of the maltose transport system in the revertant appear to form a substrate recognition site. It is likely that this site exists in wild type cells but is available only to substrate molecules that are bound to the maltose-binding protein. A model for the operation of the transport system is presented. In this model, the substrate recognition site in the cytoplasmic membrane is exposed to alternate sides of the membrane.