Mot mutants of Escherichia coli are paralysed: their flagella appear to be intact but do not rotate. The motA and motB gene products are found in the cytoplasmic membrane; they do not co-purify with flagellar basal bodies isolated in neutral detergents. Silverman et al. found that mot mutants could be ' resurrected ' through protein synthesis directed by lambda transducing phages carrying the wild-type genes. Here, we have studied this activation at the level of a single flagellar motor. Cells of a motB strain carrying plasmids in which transcription of the wild-type motB gene was controlled by the lac promoter were tethered to a glass surface by a single flagellum. These cells began to spin within several minutes after the addition of a lac inducer, and their rotational speed changed in a series of equally spaced steps. As many as 7 steps were seen in individual cells and, from the final speeds attained, as many as 16 steps could be inferred. These experiments show that each flagellar motor contains several independent force-generating units comprised, at least in part, of motB protein.