  The effect of magnetic fields on the frequencies of toroidal oscillations of
neutron stars is derived to lowest order. Interpreting the fine structure in
the QPO power spectrum of magnetars following giant flares reported by
Strohmayer and Watts (2006) to be "Zeeman splitting" of degenerate toroidal
modes, we estimate a crustal magnetic field of order 10^{15} Gauss or more. We
suggest that residual m, -m symmetry following such splitting might allow
beating of individual frequency components that is slow enough to be observed.
