We present experimental results for the lattice parameter and the thermal expansion of a silicon monocrystal at room temperature. In the precision determination of the Avogadro constant, an accurate knowledge of the value of the thermal expansion of silicon monocrystals is necessary so that their unit cell and molar volumes can both be referred to the same temperature. Recent advances in x-ray interferometry have made it possible to reduce the uncertainty of that value by one order of magnitude. (C) 1997 American Institute of Physics.