A reverse (water-in-oil) cubic phase was observed in a ternary system consisting of an amphiphilic diblock copolymer (EO(17)BO(10), where EO represents ethylene oxide and BO represents butylene oxide), water, and p-xylene in the following composition range : 47-62 wt % polymer and 7-12 wt % water. This cubic phase occurs between a reverse hexagonal liquid crystalline (H-2) and a reverse micellar solution (L(2)) region and can be considered the result of crystallization of the reverse micelles as they swell with increasing water content. Small-angle X-ray spectra from samples of this cubic phase can be indexed to the crystallographic space group Fd3m (Q227). This is one of the first times a cubic structure consisting of distinct reverse micelles has been observed in a ternary amphiphile-water-oil system; bicontinuous reverse cubic structures, such as the Gyroid (Ia3d, Q230), are more common and have been previously identified in such ternary systems between the lamellar (L(alpha)) and the H-2 phases.