EFFECTIVE-FIELD THEORY OF AN INTERNAL STRING

We obtain an effective field theory of an internal string by construct ing a gauge theory of the Virasoro-Kac-Moody symmetry associated with P circle times G, where P is the Poincare group and G is the grand uni fication group. The theory automatically breaks the Virasoro-Kac-Moody group down to P circle times H circle times U(1), where H is a subgro up of G and U(1) is the Cartan subgroup of the Virasoro group. After t he spontaneous symmetry breaking the mass spectrum of the theory is ch aracterized by two completely different mass scales: the Planck scale which is responsible for the harmonic oscillator spectrum of the strin g and the elementary particle scale which is responsible for the fine structure of the mass spectrum. The theory allows a straightforward su persymmetric generalization.