Abstract: The open mouth of a container such as a bag or sack which is filled with produce or other goods is folded by means of a single oscillatory finger which forms a fold in the leading and trailing sides of the bag as it travels along a path, as by being carried on a conveyor. The finger is oscillated by means of a mechanism driven by the bag which causes the finger to execute a forward stroke of oscillation whereby to form the fold in the leading side of the bag and then to form the fold in the trailing side of the bag as the finger executes its return stroke. The mechanism provides, as by means of a cam which oscillates with the finger, the timing of the forward and return strokes in coordination with the movement of the bag such that the bag may move continuously during the form folding operation. The fold is maintained by applying lateral pressure on opposite sides of the fold, as by means of opposed belts which engage the bag as the bag leaves the vicinity of the finger.