An optical beating technique was applied to the study of thermal fluctuations in isotropic phases of nematogens (those liquids changing into nematic liquid crystals in cooling) by Brillouin scattering. Wide-range and high-resolution spectroscopy was made. With this method, the power spectra of the light scattered in 6 CB (p-n-hexyl p[sup ']-Cyanobiphenil) was observed over the temperature range from 28.8 to 65.0 (degrees)C. Not only the fluctuation of density, which causes the Brillouin triplet, but also the fluctuation of orientation, a kind of the local order, appears in these liquids. The light-scattering spectrum reflects the reorientational relaxation. The model suggested by de Gennes theoretically predicts the spectra of both polarized (VV) and depolarized (VH) scattering. By fitting these equations to the experimental results, characteristic values were obtained which describe the dynamics of nematogens: (Gamma), the relaxation frequency of the orientational fluctuation, and C, the coupling constant between the orientational fluctuation and the local shear flow. The physical meanings of these parameters are discussed in the framework of critical phenomena.