Abstract: A rotary unit, of an FFS (Form, Fill and Seal) machine, for ultrasound sealing a continuous tubular strip housing an orderly succession of products and fed along a feed path extending through a sealing station; the rotary unit having two substantially identical rotors located on opposite sides of the feed path, and which rotate in opposite directions about respective axes, and have respective pitch surfaces tangent to each other and to the feed path; and an ultrasound sealing head, forming part of one of the two rotors, cooperates at the sealing station with a corresponding anvil forming part of the other rotor, to close the tubular strip along a transverse seal line located at a respective portion of the tubular strip extending between two respective adjacent products.