We have performed synchrotron x-ray scattering experiments on single crystals of ErNi 2 B 2 C. Below 6.3 K, the (2,0,0) Bragg peak splits, indicating a tetragonal-to-orthorhombic distortion. The mismatch between the a and b lattice parameters appears coincident with the onset of long-range antiferromagnetic order and increases continuously with decreasing temperature, reaching a value of a/bϪ1Ϸ0.2% at 3.7 K.