Heat exchanger between a first fluid and a second fluid, said exchanger comprising: - a heat exchange core for a heat exchange between the first and the second fluids, comprising first circulation channels of the first fluid and second circulation channels of the second fluid between said first channels, - an inlet collector box for the first fluid and an outlet collector box for the first fluid, into which the ends of said first channels open out, and - an inlet connection piece for the second fluid and an outlet connection piece for the second fluid, - said inlet and outlet collector boxes for the first fluid being fixed on a first side of said core and being separated by a deflector and said exchanger moreover comprising an intermediate collector box communicating with said first channels to create a U-circulation of the first fluid in said first channels, - said exchanger comprising at least one wall fixed on a second side of said core adjacent to said first side and said inlet and outlet connection pieces for the second fluid being disposed at one end of said wall, being separated one from the other, and said exchanger comprising an intermediate compartment communicating with said second channels for a U-circulation of the second fluid in said second channels, - said intermediate collector box and said intermediate compartment being disposed on sides (3d, 3e) of said core perpendicular one to the other, characterized in that said core comprises a stack of tubes (15, 15a, 15b, 15', 15"), of which at least two tubes (15', 15") are joined in such a manner that: - the lateral surfaces of said joined tubes (15', 15") are located substantially opposite the junction between said connection pieces (9, 11), and that - said deflector separating said collector boxes (5, 7) is located substantially opposite the junction between said joined tubes (15', 15"). - a heat exchange core for a heat exchange between the first and the second fluids, comprising first circulation channels of the first fluid and second circulation channels of the second fluid between said first channels, - an inlet collector box for the first fluid and an outlet collector box for the first fluid, into which the ends of said first channels open out, and - an inlet connection piece for the second fluid and an outlet connection piece for the second fluid, - said inlet and outlet collector boxes for the first fluid being fixed on a first side of said core and being separated by a deflector and said exchanger moreover comprising an intermediate collector box communicating with said first channels to create a U-circulation of the first fluid in said first channels, - said exchanger comprising at least one wall fixed on a second side of said core adjacent to said first side and said inlet and outlet connection pieces for the second fluid being disposed at one end of said wall, being separated one from the other, and said exchanger comprising an intermediate compartment communicating with said second channels for a U-circulation of the second fluid in said second channels, - said intermediate collector box and said intermediate compartment being disposed on sides (3d, 3e) of said core perpendicular one to the other, - the lateral surfaces of said joined tubes (15', 15") are located substantially opposite the junction between said connection pieces (9, 11), and that - said deflector separating said collector boxes (5, 7) is located substantially opposite the junction between said joined tubes (15', 15").