The ratio,δ, of U238 to U235 fissions was measured for various natural uranium fuel assemblies in D2O-moderated reactor lattices. Fuel element types studied included clusters of from one to nine 1-in. rods, tubular assemblies, and plate assemblies. The measurements were made by comparing the activations of depleted and natural uranium foils after irradiating them inside the fuel assemblies in an exponential facility. As a corollary to these experiments, relative decay rates were determined for the gamma activities of the U238 and U235 fission products.