We present a three-dimensional micropositioner using the inertial slider principle which is used to position an optical fiber in an atomic force microscope. It uses only two moving parts as the sideways and vertical motions are realized by either moving a cylinder along its axis or rotating it around its axis and translating the rotation into an approximately vertical motion. The device operates reliably in a baked ultrahigh vacuum system, allows positioning with sub-mu m accuracy, and has a forward range of 11.3 mm, a sideways range of 5 mm, and a vertical range of approximately 5 mm. The measured speeds without extra load fall in the range between 1.6 and 3.3 mm/min, in good agreement with the amplitude and curve shape of the applied drive signal. The minimal step size allowing consistent motion is below 25 nm. (C) 1994 American Institute of Physics.