A sideband on the low energy side of the local-vibrational-mode main absorption band of carbon acceptor in GaAs was observed. The difference between the peak frequency of the main absorption band and that of the sideband is 3.5 cm(-1). The integrated absorption of the sideband varies with temperature and has an effect on the temperature dependence of the main absorption band. It is likely that the sideband originates from the anharmonicity of the local vibrational mode itself.