The low-frequency spectrum of homeotropically aligned 4-octyl-4'-cyanobiphenyl in the smectic-A (Sm-A) phase was obtained by using optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The absence of a narrow band at 9 cm(-1), corresponding to a pseudo-lattice vibration propagating in a direction perpendicular to the smectic layers, suggests that the spectrum of homeotropically aligned Sm-A is due to orientational modes associated with rotation about the long axis of the molecule and translational modes associated with motion in the smectic layers. The spectrum is described by a bimodal lineshape function in which the lower and higher frequency components are attributed respectively to the translational and orientational modes. (C) 2003 Elsevier Science B.V. All rights reserved.