Large-amplitude oscillatory shear was applied to a poly(styrene-b-ethylenepropylene) diblock copolymer and also to binary blends of this copolymer with homopolystyrene. The shearing was applied at a temperature above the glass transition and well below the microphase separation transition for long times : 12 h, strain amplitude gamma = 40%, frequency omega = 1 rad/s, T = 150 degrees C. The dynamic moduli decrease during the shearing and after a partial recovery upon annealing at 150 OC display a permanently altered rheological state. Small-angle X-ray scattering studies indicate that the shearing induces a biaxial texture consisting predominantly of a "parallel" orientation which coexists with a nearly "transverse" orientation of lamellae, inclined at similar to 80 degrees to the parallel orientation. The shearing also causes a similar to 4% decrease in the lamellae spacing of the parallel orientation relative to the unsheared equilibrium state; lamellae in the nearly transverse orientation retain the equilibrium spacing. This lamellar contraction compares favorably to a recent theoretical treatment by Williams and MacKintosh [Macromolecules 1994, 27, 7677]. Upon annealing, the spacing of the lamellae in the parallel orientation expands to recover the equilibrium spacing, and the biaxial texture disappears via the loss of the nearly transverse orientation. After annealing, subjecting the sample to large-amplitude oscillatory shear a second time re-creates the biaxial texture.