The incoherent inelastic neutron scattering spectrum of solid n-octadecane below 600 cm(-1) is presented and compared with the results of a scaled HF ab initio calculation. Ten vibrations in the 300-550 cm(-1) region, eight of which have not been previously observed, are easily assigned as longitudinal acoustic modes (LAMs). On the basis of the good agreement between calculated and observed modes in this region, the location of the lower frequency LAMs is postulated.