A form and seal unit (1) for producing aseptic sealed packages from a tube (2) of packaging material filled with a pourable food product; the unit having a fixed structure (3), and two forming assemblies (5,5') interacting alternately and cyclically with the tube (2) of packing material, and in turn having respective pairs of jaws (7) movable between an open position and a closed position in which the jaws grip the tube (2) of packaging material between respective sealing members (13,14), and respective forming flaps (21) having respective half-shell forming portions (22) which surround the tube (2) of packaging material to determine the shape and volume of the packages; the closing movement of the forming flaps (21) about the tube (2) of packaging material is controlled by cams (30) carried by the forming flaps (21) and interacting with rollers (64) fixed to the structure (3) of the unit (1).