A multichannel flat tube for a heat exchanger has a plurality of parallel flow channels aligned in a row, side by side, along a transverse axis of the tube. At least the inner flow channels have oval cross-sections, and major semiaxes of the inner flow channels are inclined relative to the transverse axis of the tube at an acute angle. The flat tube can be designed for high operating pressures with relatively low weight and relatively high heat exchange capability. The construction may be used, for example, in CO2 air conditioners in motor vehicles.