Using a fully oriented sample of a diblock copolymer, poly(dimethylsiloxane)-block-poly-(ethylene-co-propylene), we have observed, by dynamic light scattering, the undulation mode of the block copolymer lamellae in the ordered phase. This mode is observable only in a particular geometry where the scattering vector is parallel to the lamellar structure. From the relaxation frequency of the undulation mode, the splay elastic modulus, K, of the lamellar structure is derived. It is shown that the elastic modulus K extrapolates to zero near the order-to-disorder temperature where the translational long-range order disappears. The "fast" mode usually observed in block copolymer melts on disoriented samples presumably corresponds to the undulation mode in an oriented sample, averaged over all possible orientations.