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We investigated the frequency-doubling properties of light beams whose transverse profile is given by the zero-order Bessel function J 0 (r), Bessel beams. Phase-matched second-harmonic generation in a KDP crystal was observed at angles usually not suited for phase matching. It is thereby demonstrated that under certain circumstances, Bessel beams can be viewed as light beams with tunable wavelength. A variety of applications in the field of nonlinear optics is expected.