Abstract: An arrangement in a bag-making machine for forming weld lines in a web continuously fed through the machine, preferably a flattened plastic film tube, comprises a cylindrical, freely rotatable drum; a driving belt which is adapted to rotate the drum and pressed against part of the circumferential surface of the drum; and a number of transverse sealing jaws which, as the drum rotates, are movable into engagement with and drivable together with the web for welding the web through a predetermined angle of drum rotation. In order to achieve a high speed of the web, preferably about 150 m/min., and high flexibility in respect of the available range of bag length, the sealing jaws are freely movable relative to the drum outside the circumferential surface thereof and controlled by a control device which is adapted to hold each sealing jaw in a stand-by-position out of engagement with the web, and successively to bring each sealing jaw into engagement with the web with a certain time delay relative to each other.