The present invention relates to a bioreactor including at least one reactor vessel in form of a plastic bag, a tray (4) for holding said at least one bag, a rocking device for limited rocking motions of the tray (4) around a rocking axis (2). According to the invention the bioreactor comprises a device (6,7,8,9) for enabling swinging of the tray (4) around a second axis (7) parallel to and distanced from the rocking axis (2).