We recorded the height of a granite fault surface as a function of position along one-dimensional profiles. We show that the profiles exhibit an ''anistropic'' scaling invariance: self-affinity. The difference between the maximum and the minimum height, and the standard deviation of the height, over a length L are proportional to L&zgr;, where &zgr;≈0.84. Other properties such as the Return Probability distribution or the Power Spectrum of the profile comfort this result. This self-affine property is in good agreement with recent works on artificial fractured surfaces. Previous studies at field scale are consistent with this concept. ¿ American Geophysical Union 1993