The present invention relates to a heat exchanger having a cooling core consisting of a plurality of parallel, heat exchanging tubes and two manifolds provided with slots, where the ends of heat exchanging tubes are inserted in fluid connection with each manifold and each slot is defined by a slot opening and parallel-running slot walls formed from deformed wall portions of the manifold. In order to improve a rigidity of construction of the heat exchanger the outer sides of said slot are deformed inwardly relative to the manifold wall, forming concavities in the manifold outer surface, the inner sides of said slot are deformed inwardly relative to the manifold wall, forming convexities in the manifold inner surface, and the convexities comprise grooves forming the slot walls in order to accommodate at least a portion of the longitudinal edges of the end of exchanging tube. A method of manufacturing such a heat exchanger manifold is also provided.