Cells employ a variety of linear motors, such as myosin1–3, kinesin4 and RNA polymerase5, which move along and exert force on a filamentous structure. But only one rotary motor has been investigated in detail, the bacterial flagellum6 (a complex of about 100 protein molecules7). We now show that a single molecule of F1-ATPase acts as a rotary motor, the smallest known, by direct observation of its motion. A central rotor of radius ∼1 nm, formed by its γ-subunit, turns in a stator barrel of radius ∼5nm formed by three α- and three β-subunits8. F1 ATPase, together with the membrane-embedded proton-conducting unit F0, forms the H+-ATP synthase that reversibly couples transmembrane proton flow to ATP synthesis/hydrolysis in respiring and photosynthetic cells9,10. It has been suggested that the γ-subunit of F1-ATPase rotates within the αβ-hexamer11, a conjecture supported by structural8, biochemical12,13 and spectroscopic14 studies. We attached a fluorescent actin filament to the γ-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 −l under high load.