  Proton decay is examined within the framework of certain SO(10) models which
have low dimension representations compatible with the recently found
constraints coming from string theory, and which use the Dimopoulos-Wilczek
mechanism to solve the doublet-triplet splitting problem. It is found that the
mass parameter that controls proton decay rate is intimately related to the low
energy data via the coupling unification condition, and that a suppression of
proton decay is achieved at the cost of large threshold effects. Furthermore,
in cases where there are states with intermediate masses, the threshold effects
further enhance proton decay. The experimental bound then severely constrains
the parameter space of these models. Some possible solutions are suggested.
