The use of individual molecules as functional electronic devices was first proposed in the 1970s (ref. 1). Since then, molecular electronics(2,3) has attracted much interest, particularly because it could lead to conceptually new miniaturization strategies in the electronics and computer industry. The realization of single molecule devices has remained challenging, largely owing to difficulties in achieving electrical contact to individual molecules. Recent advances in nanotechnology, however, have resulted in electrical measurements on single molecules(4-7). Here we report the fabrication of a field-effect transistor-a three-terminal switching device-that consists of one semiconductings(8-10) single-wall carbon nanotube(11,12) connected to two metal electrodes. By applying a voltage to a gate electrode, the nanotube can be switched from a conducting to an insulating state. We have previously reported(5) similar behaviour for a metallic single-wall carbon nanotube operated at extremely low temperatures. The present device, in contrast, operates at room temperature, thereby meeting an important requirement for potential practical applications. Electrical measurements on the nanotube transistor indicate that its operation characteristics can be qualitatively described by the semiclassical band-bending models currently used for traditional semiconductor devices. The fabrication of the three-terminal switching device at the level of a single molecule represents an important step towards molecular electronics.