A freezer for fluids inside bags comprises a freezing chamber (11) containing a plurality of surfaces (13) for storing bags (20). Each surface in turn comprises a refrigerated shelf (14) and an associated uniformity plate (15) arranged above the shelf. The plates (15) are hinged so as to be able to be raised from the respective shelf in order to store bags between the shelf and the plate. At least one of the uniformity plates (15) is provided with a temperature sensor (18) for checking the temperature reached by the bags on the shelf.