Proton decay in three-generation matter-parity-invariant superstring models

Proton decay in three-generation superstring models with intermediate scale is investigated. It is shown that matter-parity ($M_2$) invariance alone does not stabilize the proton. However, models with $M_2$ invariance and Calabi-Yau manifolds such that the exotic C-odd quarks and leptons are superheavy will generally lead to proton lifetimes consistent with existing data.