  We show that type I string theory compactified in four dimensions in the
presence of constant internal magnetic fields possesses N=1 supersymmetric
vacua, in which all Kahler class and complex structure closed string moduli are
fixed. Furthermore, their values can be made arbitrarily large by a suitable
tuning of the quantized magnetic fluxes. We present an explicit example for the
toroidal compactification on T^6 and discuss Calabi-Yau generalizations. This
mechanism can be complementary to other stabilization methods using closed
string fluxes but has the advantage of having an exact string description and
thus a validity away from the low-energy supergravity approximation. Moreover,
it can be easily implemented in constructions of string models based on
intersecting D-branes.
