We prove that the proton is stable in the gauge model [SU(6)]3 x Z3 wh
ich unifies nongravitational forces with flavors, broken spontaneously
by a minimal set of Higgs fields and vacuum expectation values down t
o SU(3)cxU(1)EM. We also compute the evolution of the gauge coupling c
onstants and show how agreement with precision data can be obtained.