Leadless pacemaker, comprising a hermetic housing, a pacing electrode disposed on a distal portion of the housing, an electronics package disposed in the housing and configured to generate and deliver pacing pulses to the pacing electrode, and a fixation mechanism, arranged on the distal portion of the housing, wherein the fixation mechanism comprises at least one deformable hook-shaped thin fixation wire having an attachment portion fixedly attached at the distal portion of the housing and a free end portion which is angled or bent with respect to the attachment portion such that it extends essentially in conformity but with a small spacing to a neighboured surface portion of the housing such that the free end portion engages with heart tissue onto which the distal portion of the housing is pressed, upon rotation of the pacemaker in that direction in which the free end portion extends from the attachment portion, and disengages upon rotation of the pacemaker in the opposite direction.
