An analysis is presented that relates the crack spacing in a brittle film on an elastic substrate to the stress in the film, its thickness and fracture toughness. This analysis differs from an earlier one presented by one of the authors in that it considers the effect of a sequential, rather than a concerted, propagation of cracks, and predicts a larger crack spacing. The validity of the present analysis was confirmed by experimental results from a model system consisting of epitaxial PrBa2Cu3O7-x films on SrTiO3 substrates. The experimentally observed relationship between the crack spacing and the film thickness was in excellent agreement with the theory. © 1992.