Abstract: The piston of a piston/cylinder unit of a belt pretensioner with a pyrotechnic gas generator is characterized in that the piston is made from a first metallic piston part (12) which is firmly connected with the cable (16) and at least one second piston part (14) made of a thermoplastic material, at least one section of which is arranged on the side of the first piston part (12) facing the gas generator and forms a sealing contact with the walls of the cylinder. This arrangement creates the constructional prerequisites for further advantageous developments of the piston as regards the safety of the belt pretensioner. In particular, the piston can be equipped with a sealing lip which releases itself from the inner walls of the cylinder of the piston/cylinder unit when the unit is heated, e.g. in a vehicle fire, and so offers an escape path for the gases in order to avoid the danger of explosion.