the fastener in a direction substantially perpendicularly to the plane of the stringer tapes 10 10 39 by the members 18 and 19 of the ultrasonic welding assembly and are further compressed from their opposite lateral sides by the pair of pressers 20 and 20 39 thus vibrated at an ultrasonic frequency the interengaged lowermost scoops 12 and 12 39 of the slide fastener are welded or fused together as at 17 in fig1 and 2 owing to the heat generated frictionally by their contacting surface portions at the same time the coupling heads 15 15 39 are fused integrally with the core cord 13 13 39 the desired bottom stop 16 is thus completed it will be appreciated that the frictional heat generated as above by the interengaged lowermost scoops 12 and 12 39 is much higher at a given ultrasonic frequency of vibration than in the case where they are not compressed from the opposite lateral sides the bottom stop 16 can therefore be formed efficiently and with a high degree of coherency as an additional advantage the interengaged lowermost scoops 12 and 12 39 are not appreciably deformed when ultrasonically welded together by the method of this invention so that the bottom stop 16 is favorable in appearance too the specific example of the inventive method shown and described hereinbefore is meant purely to illustrate or explain.