  We study the constraints from the $b{\rightarrow}s{\gamma}$ decay in the
parameter space of effective supergravities from orbifold string theory and
with minimal supesymmetric particle content. Both the general dilaton-dominated
universal scenario as well as a non-universal scenario for the soft terms are
investigated. It is found that the recently reported CLEO upper and lower
bounds constrain the parameter space of the models under scrutiny. In
particular we find constraints on the values of the parameter $\tan{\beta}$ and
the gluino masses. In this class of string scenarios the negative sign of the
Higgs mixing parameter $\mu$, is phenomenologically preferred.
