Extended x-ray absorption fine structure has been used to determine a crystallographic distortion in the Co layers in Co-Cu single-crystal superlattices grown on Cu(100). The FCC structure of the superlattice imposed by the Cu substrate is distorted along the growing direction for the Co layers, which am compressed perpendicularly to the interfaces. They adopt the in-plane lattice constant of the Cu substrate, keeping constant the atomic volume of Co (11.30 angstrom3/atom), close to the value for HCP-Co.