Crossed-progeny fueling systems are those in which a reactor, such as a PWR, would burn 233U in natural uranium and supply plutonium to a D2O reactor, which in turn would burn the plutonium in thorium and supply 233U for the PWR. Optimum moderator-to-fuel ratios are determined for these systems. They are influenced noticeably by plutonium credits but little by processing costs. Conversion ratios, neutron balances, and fuel cycle costs are calculated. Uranium-233 in natural uranium is an attractive fuel in a PWR, but plutonium-in-thorium, which does not appear attractive in a PWR, does appear so in a D2O-moderated reactor.