An ultrasonic sealing device for a top fin of a gable-topped carton includes two jaws consisting of a jaw (2) with a planar active face and constituting the outer end portion of an ultrasonic horn (A1) and a jaw (6) constituting an anvil and having an active surface (10) which is planar except for upper and lower linear ribs (12) and (14) each extending the length of the surface (10). The lower rib (14) is linear and horizontal except for an upwardly bowed middle portion (16). The two jaws (2) and (6) serve to render gas-and-liquid tight a top fin of a conventional, plastics-coated paperboard, gable-top carton. The ribs (12) and (14) produce, in the outer surface of one of the outer sealing panels of the fin, corresponding furrows which extend over the whole length of the fin at levels where the inner sealing panels are sandwiched between the outer sealing panels, so as to seal the top fin in a gas- and liquid-tight manner along its whole length.