Twitching and social gliding motility allow many gram negative bacteria to crawl along surfaces, and are implicated in a wide range of biological functions. Type IV pili (Tfp) are required for twitching and social gliding, but the mechanism by which these filaments promote motility has remained enigmatic. Here we use laser tweezers to show that Tfp forcefully retract. Neisseria gonorrhoeae cells that produce Tfp actively crawl on a glass surface and form adherent microcolonies. When laser tweezers are used to place and hold cells near a microcolony, retractile forces pull the cells toward the microcolony. In quantitative experiments, the Tfp of immobilized bacteria bind to latex beads and retract, pulling beads from the tweezers at forces that can exceed 80 pN. Episodes of retraction terminate with release or breakage of the Tfp tether. Both motility and retraction mediated by Tfp occur at about 1 microm s(-1) and require protein synthesis and function of the PilT protein. Our experiments establish that Tfp filaments retract, generate substantial force and directly mediate cell movement.