  We find that the anomalous dimension (in the $\bar{MS}$ scheme) of the
chromo-magnetic operator $g_s\bar h_v\sigma_{\mu\nu} G^{\mu\nu} h_v$ in the
heavy-quark effective theory is \gamma_{mag} = (C_A\alpha_s / 2\pi) [1 + (
17/18 C_A - 13/18 T_F n_f)\alpha_s/\pi + O(\alpha_s^2)] generalizing the
one-loop expression known previously. To derive the two-loop result, we use the
reparametrization invariance and the virial theorem. After performing infrared
subtractions, all two-loop integrals are of propagator type and are evaluated
by a recurrence relation for tensor integrals.
