Abstract: A corrugated fin or turbulizer for a heat exchanger comprises a series of corrugations with parallel side walls. The side walls are provided with a series of parallel slits between which one-sided or two-sided louvers are defined. Each of the louvers has first and second edges extending along an adjacent pair of slits, and at least one bend located between the edges, thereby causing at least one of the edges of the louver to project outwardly of the plane of the side wall, and providing the side wall with improved crush resistance. The corrugations may preferably be rectangular or trapezoidal in form, having generally flat top and bottom surfaces defined by two closely-spaced bends. The top and bottom surfaces may preferably be provided with protrusions, at least some of which extend close to the bends. This assists in creating localized areas of weakness along which the bends can be formed cleanly.