Caulobacter crescentus flagellar filaments were examined for their shape and handedness. Contour length, wavelength and height of the helical filaments were 1.34 +/- 0.14 micron, 1.08 +/- 0.05 micron and 0.27 +/- 0.04 micron, respectively. Together with the value of the filament diameter, 14 +/- 1.5 nm, the parameters of the curvature (alpha) and twist (phi) were calculated as 3.9(%) for alpha and 0.026 (rad) for phi, which are similar to those of the curly I filament of Salmonella typhimurium. Dark-field light microscopic analysis revealed that the C. crescentus wild-type filament possesses a right-handed helical form. Given the result that C. crescentus cells normally swim forward, in the opposite direction to a polar flagellum, it is likely that C. crescentus swims by rotation of a right-handed curly shaped flagellum in a clockwise sense, whereas S. typhimurium and Escherichia coli swim by rotation of left-handed normal type flagella in a counterclockwise sense.