Strong coupling unification and extra dimensions

We analyze the implications of electroweak and strong coupling unification in a very general class of models extending the minimal supersymmetric stan dard model in 4 + p dimensions (p greater than or equal to 0). In general, electroweak precision data require the presence of large extra dimensions ( low compactification scales, M-c) and/or low unification scale, M-U. In par ticular, the actual experimental value of the strong coupling at M-Z impose s an upper bound on the compactification and unification scales. In four-di mensional theories (M-c similar or equal to M-U) with canonical hypercharge assignment we find M-U less than or similar to 10(9) GeV. In theories with extra dimensions (M-c < M-U) we find M-c less than or similar to 10(7) GeV , for a supersymmetric spectrum at the TeV scale. (C) 1999 Elsevier Science B.V. All rights reserved.