Bacterial cells possess ultra-small motors on their surfaces with which to rotate their flagellar filaments. The motor utilizes the electrochemical energy stored in the proton gradient across the cytoplasmic membrane, and can rotate at more than 200 r.p.s without a load. It can rotate in both clockwise and counterclockwise directions and switch the rotational direction in 1 msec. Its rotator is made of about 10 kinds of proteins and is about 30 nm in diameter. To analyze the motor function in detail, the authors have developed a laser dark-field microscopy technique by which high-speed rotation of a single flagellum can be measured. They have also succeeded in controlling the rotation speed by applying an external electric pulse to a bacterial cell that is held at the tip of a micropipette.