Apparatus for filling bags (1,1a,1b) having a mouth extending in a direction of width with loose material, comprising: - grippers (112,112a) for gripping the opposite sides of the bag, which are arranged opposite each other relative to a vertical axis (Z-Z) of symmetry and are movable symmetrically towards/away from each other with respect to said axis in a direction (Y-Y; X-X) parallel to the direction of width of the bag; - suction cups for gripping the side portions of the bag, movable symmetrically towards/away from each other along a direction (X-X;Y-Y) perpendicular to the direction of width of the bag for opening/gripping the mouth of the bag, - valves rotating from a closing position outside the bag into an opening position inside the mouth of the bag, wherein the apparatus comprises: a pair of first carriages arranged opposite each other and movable, in the direction (Y-Y;X-X) of width of the bag symmetrically with respect to the vertical central axis (Z-Z) of symmetry, from a minimum/maximum spaced position to a maximum/minimum spaced position, wherein the following are fixed to each first carriage: - a flange extending in the direction of width of the bag towards the axis of symmetry (Z-Z) and designed to form an end-of-travel stop surface for rotation of the valves of the bag-holder device, - said grippers (112;112a) for gripping the opposite edges of the bag, and wherein the initial relative distance with respect to the axis of symmetry (Z-Z) between the end-of-travel flanges and the gripping grippers is constant and independent of the width of the bag, characterized in that said flange of each first carriage is designed to form an adjustable stop surface for end-of-travel of rotation for opening and entry into the bag mouth of the valves of the bag-holder device, and in that the movements for positioning the two first carriages, the end-of-travel flanges for the rotation of the valves and the grippers (112,112a) for gripping the edges of the bag are coordinated and performed by means of a single start-of-cycle command depending on the widthwise dimension of the bag. - grippers (112,112a) for gripping the opposite sides of the bag, which are arranged opposite each other relative to a vertical axis (Z-Z) of symmetry and are movable symmetrically towards/away from each other with respect to said axis in a direction (Y-Y; X-X) parallel to the direction of width of the bag; - suction cups for gripping the side portions of the bag, movable symmetrically towards/away from each other along a direction (X-X;Y-Y) perpendicular to the direction of width of the bag for opening/gripping the mouth of the bag, - valves rotating from a closing position outside the bag into an opening position inside the mouth of the bag, a pair of first carriages arranged opposite each other and movable, in the direction (Y-Y;X-X) of width of the bag symmetrically with respect to the vertical central axis (Z-Z) of symmetry, from a minimum/maximum spaced position to a maximum/minimum spaced position, wherein the following are fixed to each first carriage: - a flange extending in the direction of width of the bag towards the axis of symmetry (Z-Z) and designed to form an end-of-travel stop surface for rotation of the valves of the bag-holder device, - said grippers (112;112a) for gripping the opposite edges of the bag, - a flange extending in the direction of width of the bag towards the axis of symmetry (Z-Z) and designed to form an end-of-travel stop surface for rotation of the valves of the bag-holder device, - said grippers (112;112a) for gripping the opposite edges of the bag, and wherein the initial relative distance with respect to the axis of symmetry (Z-Z) between the end-of-travel flanges and the gripping grippers is constant and independent of the width of the bag,