Abstract We derive from a superstring a simple GUT with direct unification of gauge coupling constants at the string scale, i.e., M x ≈ 10 18GeV ≈ M string, irrespective of the string threshold effects, by fermionic construction. It is a D-parity violated supersymmetric SU(4) × SU(2) L × SU(2) R (Pati-Salam) model which breaks to the supersymmetric standard model at an intermediate sca M R and a massless spectrum which contains at M X the three generations of quarks and leptons, two Higgs multiplets in the representation ( 4, 1, 2) + ( 4 , 1, 2) plus one Higgs multiplet in ( 1, 2, 2). By identifying M X with the effective string unification scale M ̂ string M string exp( π 6 ) in which a small string threshold correction has been taken into account, the mass scales can be selfconsistently fixed as M X ≈ 6.3 × 10 17 GeV, M R ≈ 6.4 × 10 11 GeV and M S (the supersymmetry breaking scale) ≈ 2.0 TeV in terms of the renormalization group equations. The scale M R might give new insight into phenomenology. For example, it produces a cosmologically interesting τ-neutrino mass in the 10 eV range owing to the see-saw mechanism with Majorana τ-neutrino mass ≈ 10 12 GeV.