The effect of shear on the alignment of the lamellar phase in a concentrated aqueous salt solution of a poly(oxyethylene) and poly(oxybutylene) diblock copolymer is investigated in situ using small-angle X-ray scattering (SAXS). The alignment of the lamellae is deduced from the anisotropy of the scattering patterns. It is found that the orientation of the lamellae with respect to the shear direction alters with an increase in the shear rate: the lamellae are oriented transverse to the shear direction at low shear rates, and perpendicular to the vorticity direction at high shear rates. The transition between the two lamellar alignments occurs at a critical shear rate in the range: 10 s(-1) < gamma(c) < 100 s(-1).