Roundbaler having: a variable bale-pressing chamber, wherein the variable bale-pressing chamber is delimited by side walls, which are arranged to the right and left in the direction of forward travel, and at least one pressing belt running around between the side walls, and at least one sensor (98, 132) which is designed as an angle of rotation sensor or displacement sensor, is in direct or indirect operative connection with the pressing belt and is intended to detect a positional change of the pressing belt in the direction of one of the side walls, wherein at least one actuating member in the form of a pivoting arm or linearly displaceable slide is arranged, via which the sensor (98, 132) is in operative connection with the pressing belt and which interacts with the sensor (98, 132), the actuating member, which is mounted on a frame part of the roundbaler, can be brought directly or indirectly into engagement with a peripheral edge of the pressing belt, and an adjustment of the actuating member that can be detected by the sensor (98, 132) can be produced by a positional change of the pressing belt, characterized in that the actuating member comprises a guide roller which can roll on the peripheral edge and is arranged on a pivotable, free end of the pivoting arm or displaceable, free end of the slide, and in that the actuating member can be moved against a first stop and a second stop, which stops (116, 118) are configured as ends of a circular arc-shaped or linearly formed guide groove (107, 123) in the frame part, which is configured as a crossmember, through which groove the one guide pin, which connects the pivoting arm to the guide roller, or a guide bolt, via which the slide is guided in the guide groove, extends and bears in its end positions, wherein the sensor (98, 132) is arranged on the side of the crossmember facing away from the pressing belt. a variable bale-pressing chamber, wherein the variable bale-pressing chamber is delimited by side walls, which are arranged to the right and left in the direction of forward travel, and at least one pressing belt running around between the side walls, and at least one sensor (98, 132) which is designed as an angle of rotation sensor or displacement sensor, is in direct or indirect operative connection with the pressing belt and is intended to detect a positional change of the pressing belt in the direction of one of the side walls, wherein at least one actuating member in the form of a pivoting arm or linearly displaceable slide is arranged, via which the sensor (98, 132) is in operative connection with the pressing belt and which interacts with the sensor (98, 132), the actuating member, which is mounted on a frame part of the roundbaler, can be brought directly or indirectly into engagement with a peripheral edge of the pressing belt, and an adjustment of the actuating member that can be detected by the sensor (98, 132) can be produced by a positional change of the pressing belt, characterized in that the actuating member comprises a guide roller which can roll on the peripheral edge and is arranged on a pivotable, free end of the pivoting arm or displaceable, free end of the slide, and in that the actuating member can be moved against a first stop and a second stop, which stops (116, 118) are configured as ends of a circular arc-shaped or linearly formed guide groove (107, 123) in the frame part, which is configured as a crossmember, through which groove the one guide pin, which connects the pivoting arm to the guide roller, or a guide bolt, via which the slide is guided in the guide groove, extends and bears in its end positions, wherein the sensor (98, 132) is arranged on the side of the crossmember facing away from the pressing belt.