A flat tube for a heat exchanger having a stack of such flat tubes. The flat tube has a plurality of spaced-apart dimples formed on each of its opposite faces. At least some of the dimples on each face have an elongate plan shape, at least some of the elongate dimples being obliquely oriented such that their major axes form oblique angles with a longitudinal axis of the tube. The obliquely oriented elongate dimples are positioned such that when two of the tubes are stacked, the obliquely oriented elongate dimples on opposing faces of the tubes contact one another and the major axes of each contacting pair of elongate dimples are non-parallel to each other. Each contacting pair of elongate dimples of adjacent tubes thus forms a “cross” or “X”.