The invention relates to a belt pretensioner for vehicle safety belt systems comprising a pyrotechnic gas generator (12) and a piston and cylinder unit (16, 20) mounted in a housing block (14). The piston (20) of the piston and cylinder unit (16, 20) is arranged on a first end (161) of the cylinder (16) as well as being connected with a cable (22) and it is accelerated towards the second end (162), which has a constriction, of the cylinder (16) by the gas pressure produced by the gas generator (12). The piston and cylinder unit (16, 20) with the housing block (14) and the gas generator (12) mounted therein constitute a pivotally bearinged vehicle-sensitive inertial mass. In accordance with the invention a deformable member (46) is arranged on the piston (20) and on acceleration of the piston (20) is entrained towards the second end (162) of the cylinder (16) by the piston (20) and on impingement on the wall parts of the cylinder (16) constituting the constriction is plastically deformed.