The 200 K protein dynamical transition is observed for the first time in the terahertz dielectric response. The complex dielectric permittivity epsilon = epsilon' + i epsilon' is determined in the 0.2-2.0 THz and 80-294 K ranges. epsilon" has a linear temperature dependence up to 200 K then sharply increases. The low temperature linear dependence in 0 suggests anharmonicity for temperatures 80 K < T < 180 K, challenging the assumed harmonicity below 200 K. The temperature dependence is consistent with thermally activated sidechain motions and shows the motions involved in the dynamical transition extend to subpicosecond time scales. (c) 2007 Elsevier B.V. All rights reserved.