A heat exchanger tube (20) of elongate cross-section has an internally located tubular stiffener (26) which prevents deflection of the wall (17) of the tube (20) due to pressure differentials between the internal (30) and external (19) surfaces of the tube (20) while allowing flow of fluid between chambers at areas on opposite sides of the tubular stiffener (26) inside the tube (20) through apertures (32) in the walls of the tubular stiffener (26). The heat exchanger tube (20) can have an elliptical, oval or flat cross-section and the internal tubular stiffener (26) can have a variety of cross-sectional configurations having uniform or non-uniform shapes.