We report light-scattering measurements of powder diffraction patterns of crystals of essentially hard colloidal spheres. These are consistent with structures formed by stacking close-packed planes of particles in a sequence of permitted lateral positions; A ; B ; C ; which shows a high degree of randomness. Crystals grown slowly; while still containing many stacking faults; show a tendency towards face-centered-cubic packing: possible explanations for this observation are discussed.