Coupling a fast and thermal assembly in a power breeder reactor affords the possibility of obtaining the relatively long neutron lifetime characteristic of a thermal assembly and the high breeding ratio characteristic of a fast assembly. General properties of such mixed systems are discussed. A suggested design is discussed and compared with a prototype all-fast system. The coupled system considered consists of a 400-liter Pu239 fueled, Na-cooled, fast core surrounded by a 10-cm inner blanket annulus containing natural U, Na coolant, and structural material, but no moderator. Outside the inner blanket is a 30-cm annulus of Be surrounded by an outer blanket consisting primarily of depleted U. The inner blanket serves as core for the thermal system, as barrier for low-energy neutrons between moderator and fast core, and as reflector for the fast core. Its construction is essentially the same as the first part of the blanket in a fast power breeder, so that the transition from an allfast system to the coupled system involves only the replacement of blanket material by moderator and the use of natural rather than depleted uranium in the inner blanket. The properties of the system described are thereby changed: neutron lifetime increases from ∼1.5 × 10-7 sec. to ∼2×10-5 sec; breeding ratio reduced ∼10%; fast core critical mass decreased ∼10%; multiplication constant of the system without the contribution of thermal fissions ∼0.95; thermal fissions generate ∼13% of total power; and the radial power distribution in fast core flattened, maximum to average ratio reduced from ∼1.5 to ∼1.3.