A bag-puckering system has a pairs of grippers engageable with upper corners of respective bags. The grippers of each pair are shiftable between an inner position in which the respective bag is puckered open and an outer position in which the respective bag is held flat. The pairs of grippers are aligned in a longitudinal row and shiftable in a travel direction through a filling station wherein fluent material can be poured into the bags. A controller upstream of the filling station measures a length in the direction of the bags and generates respective outputs corresponding thereto. A movable cam at the filling station operatively engages the grippers in the filling station for shifting the grippers between their inner and outer positions. A drive connected to the control means and to the cam can shift the grippers in accordance with the size of the respective bags.