  The supersymmetric particle spectrum is calculated in type I string theories
formulated as orientifold compactifications of type IIB string theory. A string
scale at an intermediate value of $10^{11}-10^{12}$ GeV is assumed and extra
vector-like matter states are introduced to allow unification of gauge coupling
constants to occur at this scale. The qualitative features of the spectrum are
compared with Calabi-Yau compactification of the weakly coupled heterotic
string and with the eleven dimensional supergravity limit of $M$-theory. Some
striking differences are observed. Assuming that the lightest neutralino
provides the dark matter in the universe, further constraints on the sparticle
spectrum are obtained. Direct detection rates for dark matter are estimated.
