  We present evidence that the CHL string in eight dimensions is dual to
F-theory compactified on an elliptic K3 with a $\Gamma_{0}(2)$ monodromy group.
The monodromy group $\Gamma_{0}(2)$ allows one to turn on the flux of an
antisymmetric two form along the base. The $B_{\mu \nu}$ flux is quantized and
therefore the moduli space of the CHL string is disconnected from the moduli
space of F-theory/Heterotic strings (as expected). The non-zero $B_{\mu \nu}$
flux obstructs certain deformations restricting the moduli of elliptic K3 to a
10 dimensional moduli space. We also discuss how one can reconstruct the gauge
groups from the elliptic fibration structure.
