Cells employ a variety of linear motors, such as myosin(1-3), kinesin(4) and RNA polymerase(5), which move along and exert force on a filamentous structure. But only one rotary motor has been investigated in detail, the bacterial flagellum(6) (a complex of about 100 protein molecules(7)). We now show that a single molecule of F-1-ATPase acts as a rotary motor, the smallest known, by direct observation of its motion. A central rotor of radius similar to 1 nm, formed by its gamma-subunit, turns in a stator barrel of radius similar to 5nm formed by three alpha- and three beta-subunits(8). F-1-ATPase, together with the membrane-embedded proton-conducting unit F-0, forms the H+-ATP synthase that reversibly couples transmembrane proton flow to ATP synthesis/hydrolysis in respiring and photosynthetic cells(9,10). It has been suggested that the gamma-subunit of F-1-ATPase rotates within the alpha beta-hexamer(11), a conjecture supported by structural(8), biochemical(12,13) and spectroscopic(14) studies. We attached a fluorescent actin filament to the gamma-subunit as a marker, which enabled us to observe this motion directly. In the presence of ATP, the filament rotated for more than 100 revolutions in an anticlockwise direction when viewed from the ’membrane’ side. The rotary torque produced reached more than 40 pN nm(-1) under high load.