The phase behavior of polyisoprene-polystyrene (PI-PS) diblock copolymers near the order-disorder transition was investigated using dynamic mechanical measurements, transmission electron microscopy, and small-angle X-ray and neutron scattering at polyisoprene volume fractions 0.33 < f(PI) < 0.42. Two new ordered morphologies have been documented between hexagonally packed cylinders (HEX) and lamellae (LAM). At low temperatures a hexagonally perforated layered (HPL) microstructure occurs, that transforms into a bicontinuous cubic phase characterized by I (a) over bar 3d space group symmetry near the order-disorder transition (ODT). The ordered-bicontinuous double diamind (OBDD) phase was not found at any temperature or composition studied. We demonstrate that the bicontinuous I (a) over bar 3d phase exists at equilibrium over a narrow range of compositions, 0.35 less than or similar to f(PI) less than or similar to 0.40 bounded by the HEX, LAM, HPL, and disordered states, and is restricted to within about 60 OC of the ODT, suggesting the importance of finite size effects. This cubic phase also exhibits pronounced metastability, remaining intact for extended periods of time after cooling below the I (a) over bar 3d-HEX or I (a) over bar 3d-HPL phase boundary. Cooling from above the ODT to below the PS glass transition temperature leads to a vitrified form of the I (a) over bar 3d state.