The localization of hook-associated proteins (HAP1, HAP2, and HAP3) in Salmonella typhimurium flagella was studied by using specific antibodies together with a second antibody conjugated with colloidal gold. HAP1 and HAP3 were localized at the hook-filament junction, as has been suggested previously. HAP2, however, was localized at the filament tip. This finding supports the idea that HAP2 acts to induce polymerization of endogenous flagellin at the filament tip, and HAP1 and HAP3 are junction proteins to connect hook with filament. Analysis of the protein composition of short flagella from a mutant indicated that a single flagellum contains about 10 to 20 HAP1, 10 to 20 HAP2, and 10 to 40 HAP3 molecules.