The scanning tunnelling microscope1 (STM) has been employed in recent years in attempts to develop atomic-scale electronic devices, both by examining device-like characteristics in preexisting structures2,3 and by creating new structures by the precise manipulation of atoms and molecules with the STM tip4-6. Here we report the operation of a bistable switch that derives its function from the motion of a single atom. A xenon atom is moved reversibly between stable positions on each of two stationary conducting 'leads', corresponding to the STM tip and a nickel surface. The state of the switch is set (that is, the xenon atom is moved to the desired location) by the application of a voltage pulse of the appropriate sign across the leads. The state of the switch is identified by measuring the conductance across the leads. This switch is a prototype of a new class of potentially very small electronic devices which we will call atom switches. © 1991 Nature Publishing Group.