Contrary to expectation, large argon clusters Ar-N, 10(3) < N < 10(5), do not adopt the bulk fee crystal structure. Rather they contain a mixture of fcc, hcp, and random close-packed regions, with no significant preference for fcc. This can be concluded from a simulation of observed electron diffraction patterns of large unsupported argon clusters with structural models. The new diffraction patterns exhibit features that have not been detected previously in experiments with smaller clusters; these features can unambiguously be attributed to hcp stacking.