The authors report the identification of a new equilibrium microdomain morphology in an intermediate to weakly segregated diblock copolymer melt. A polystyrene-polyisoprene (SI) diblock copolymer consisting of 37 wt% styrene and of total M[sub w] = 27,400 was observed to transform from the lamellar morphology (in equilibrium at low annealing temperatures) to a new morphology at annealing temperatures approximately 50 C below the order-disorder transition (ODT). The transformation was observed to be thermoreversible. Investigation of the new morphology via small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) revealed the new structure to have remarkable three-dimensional long-range order, to belong to the cubic space group Ia3d, and to possess a bicontinuous cubic microstructure. From computer simulations of model structures and comparison with microscopy results, the authors propose models for the new morphology based on the triply periodic G minimal surface (gyroid) discovered by Schoen; similar morphologies have been observed in a variety of microphase-separated surfactant-water systems. Blends of this diblock with various short-chain homopolymers were used to investigate the compositional extent of the region of Ia3d stability on the phase diagram; the results indicate that the Ia3d phase is stable over a wide range of minority component volume fractions.