Abstract: Heat exchangers are known which are constructed of flat tubes which are formed in that the bent-open tube ends of the flat tubes are joined with contact surfaces, openings in the contact surfaces contributing to the formation of coolant tanks. The closure of the bent-open areas is achieved by inserting special covers. The bent-open areas of the tube ends are constructed to be larger and to use the wall parts projecting beyond the contact surface for forming a tight closure. This can take place by a folding, an overlapping or an obtuse placing against one another of the wall parts which are folded to the inside. The flat-tube heat exchangers may be used, for example, as transmission oil coolers.