[en] We report the first successful measurement of a magnetic X-ray powder diffraction (MXPD) peak from an antiferromagnet, obtained in this instance by exploiting the strong uranium M₄ magnetic resonance in UO₂. In the experiment, the challenge has been to overcome the relative weakness of MXPD relative to high levels of fluorescence background radiation. The interpretation of our data as magnetic diffraction rests on three findings. First, the signal vanishes as the sample temperature is raised through the critical temperature, at which long-range order vanishes. Secondly, the diffraction intensities are shown to exhibit a pronounced resonance structure close to the U M₄ edge. Lastly, the scattering cross-section is found to be in remarkably good agreement with a simple calculation. (author). Letter-to-the-editor