We discuss the impact of horizontal gauge symmetries on nucleon decay and demonstrate that the ongoing proton-decay experiments may be used to distinguish between different horizontal gauge theories. Under very general assumptions, it is shown that although a minimal U(1)H horizontal symmetry allows dimension-six operators and leads to the decay of the nucleon into leptons of the first generation only, an SU(3)H horizontal symmetry forbids any baryon-nonconserving operator of dimension less than nine. © 1984 The American Physical Society.