The MotB protein of Escherichia coli is an essential component in each of eight torque generators in the flagellar rotary motor. Based on its membrane topology, it has been suggested that MotB might be a linker that fastens the torque-generating machinery to the cell wall. Here, we report the isolation and characterization of a number of motB mutants. As found previously for motA, many alleles of motB were dominant, as expected if MotB is a component of the motor. In other respects, however, the motB mutants differed from the motA mutants. Most of the mutations mapped to a hydrophilic, periplasmic domain of the protein, and nothing comparable to the slow-swimming alleles of motA, which show normal torque when tethered, was found. Some motB mutants retained partial function, but when tethered they produced subnormal torque, indicating that their motors contained only one or two functional torque generators. These results support the hypothesis that MotB is a linker.