Fermions without vierbeins in curved space-time

A general formulation of spinor fields in Riemannian space-time is given without using vierbeins. The space-time dependence of the Dirac matrices required by the anticommutation relation {gamma (mu), gamma (nu)} = 2g(mu nu) determines the spin connection. The action is invariant under any local spin base transformations in the 32 parameter group G1(4,c) and not just under local Lorentz transformations. The Dirac equation and the energy-momentum tensor are computed from the action.