In Escherichia coli, the binding protein-dependent transport system for maltose and maltodextrins is composed of five proteins--LamB, MalE, MalF, MalG and MalK--located in the three layers of the bacterial envelope. Proteins MalF and MalG are hydrophobic inner membrane components mediating the energy-dependent translocation of substrates into the cytoplasm. In this paper, we analyse the topology of the MalG protein by using methods based on the properties of fusions between malG and 'phoA, a truncated gene encoding alkaline phosphatase lacking its translation initiation and exportation signals. Fusions were obtained by using either phage lambda TnphoA or by constructing in vitro fusions located randomly within the malG gene. The deduced topological model suggests that MalG spans the membrane six times and has its amino- and carboxy-termini in the cytoplasm. These results will be helpful for the interpretation of the phenotypes of mutants in malG.