A tube for a heat exchanger, comprising a tube outer body including first and second broad, opposing, spaced apart, planar walls joined by first and second arcuate end walls, walls, the tube outer body enclosing a tube inner volume bounded by the first and second broad planar walls and the first and second arcuate end walls; a corrugated insert received within the tube inner volume, the corrugated insert including planar flanks joined by crests and troughs, the crests and troughs being bonded to the first and second broad planar walls, respectively; a plurality of flow channels extending through the tube inner volume from an inlet end of the tube to an outlet end of the tube, each one of the plurality of flow channels being bounded by one of the first and second broad planar walls of the tube outer body, a crest or a trough of the corrugated insert, and two adjacent ones of the planar flanks that are joined by that crest or trough, wherein adjacent ones of the plurality of flow channels are in fluid communication with one another between the inlet and outlet ends of the tube by way of a plurality of openings in the planar flanks; a first bypass channel bounded by at least one of the first and second broad planar walls, the first arcuate end wall, and a first end one of the planar flanks, the first end one of the planar flanks being that one of the planar flanks arranged nearest to the first arcuate end wall; and a second bypass channel bounded by at least one of the first and second broad planar walls, the second arcuate end wall, and a second end one of the planar flanks, the second end one of the planar flanks being that one of the planar flanks arranged nearest to the second arcuate end wall; characterised in that the first and second end ones of the planar flanks are absent of any openings, so that flow between any one the plurality of flow channels and either of the first and second bypass channels (11,12) is prevented. a tube outer body including first and second broad, opposing, spaced apart, planar walls joined by first and second arcuate end walls, walls, the tube outer body enclosing a tube inner volume bounded by the first and second broad planar walls and the first and second arcuate end walls; a corrugated insert received within the tube inner volume, the corrugated insert including planar flanks joined by crests and troughs, the crests and troughs being bonded to the first and second broad planar walls, respectively; a plurality of flow channels extending through the tube inner volume from an inlet end of the tube to an outlet end of the tube, each one of the plurality of flow channels being bounded by one of the first and second broad planar walls of the tube outer body, a crest or a trough of the corrugated insert, and two adjacent ones of the planar flanks that are joined by that crest or trough, wherein adjacent ones of the plurality of flow channels are in fluid communication with one another between the inlet and outlet ends of the tube by way of a plurality of openings in the planar flanks; a first bypass channel bounded by at least one of the first and second broad planar walls, the first arcuate end wall, and a first end one of the planar flanks, the first end one of the planar flanks being that one of the planar flanks arranged nearest to the first arcuate end wall; and a second bypass channel bounded by at least one of the first and second broad planar walls, the second arcuate end wall, and a second end one of the planar flanks, the second end one of the planar flanks being that one of the planar flanks arranged nearest to the second arcuate end wall; characterised in that the first and second end ones of the planar flanks are absent of any openings, so that flow between any one the plurality of flow channels and either of the first and second bypass channels (11,12) is prevented.