It is demonstrated that hydrogen can migrate in silicon as a negatively charged species (${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$). The evidence is the combined observation of a strong electric-field dependence in the rate of removal of PH complexes during bias-temperature stress of hydrogenated Schottky-barrier diodes and the resulting spatial redistribution of neutralized donors. The detection of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ establishes that, in addition to the previously determined deep-donor level, there also exists an acceptor level for hydrogen in the silicon band gap. The PH dissociation kinetics yield an activation energy of 1.18 eV, from which we estimate a binding energy in the range of 0.35 to 0.65 eV.