Nine temperature-sensitive nonflagellate mutants defective in flaFV were isolated from a strain of Salmonella typhimurium. Among them three mutants were found to produce flagella with abnormally shaped (either straight or irregularly curved) hooks at the permissive temperature. Two mutations that rendered hooks straight were located in one of the eight segments of flaFV defined by deletion mapping. The mutation that rendered hooks irregularly curved was located in a different segment. An flaR mutation was introduced into the latter mutant. At the permissive temperature, the resulting double mutant produced polyhooks whose wavelength and amplitude were both exceedingly reduced. These polyhook structures were more thermolabile than those of the flaFV+ strain. Hook protein of the former strain was shown to have a slightly positive electric charge compared with that of the latter. From these results and other available information, it is inferred that flaFV is the structural gene for the hook protein in Salmonella.