The basic structure of the shaft of the bacterial flagellum is described and an account is given of work on polymorphism to date. The flagellum of wild-type Salmonella SJ670 has a characteristic left-hand helical form from pH 8 to pH 6, but undergoes two sharp transitions to other definite forms as the pH is lowered. The first transition, to a tight coil, can be followed by flow-birefringence. The flow-birefringence curve shows no evidence of hysteresis. Other details of the polymorphism, including the transitions which occur at high pH and at various KCl concentrations, are shown in a 'phase diagram'. Details of the transformation were studied by observing individual reconstituted flagella under dark-field light microscopy, while varying the bathing medium. Under some conditions, stress due to flow of the medium influences the transition. Under constant flow, alteration between two helical forms of opposite hand was sometimes observed. The two-state model is introduced to account for the helical structure and the polymorphism. Its prediction, that the two possible straight forms should have definite and opposite values of twist, is shown to agree with optical diffraction studies. Certain straight mutants are mentioned which show a novel feature in the diffraction pattern, probably due to a periodic perturbation of the helical lattice. The co-polymerisation of flagellins from straight mutants of the two different types yields a range of forms similar to the natural polymorphs. The implications of this finding are discussed.