  We present the first direct determination of a stellar metallicity in the
spiral galaxy NGC4258 (D=7.6 Mpc) based on the quantitative analysis of a
low-resolution (~5 AE) Keck LRIS spectrum of a blue supergiant star located in
its disk. A determination of stellar metallicity in this galaxy is important
for the absolute calibration of the Cepheid Period-Luminosity relation as an
anchor for the extragalactic distance scale and for a better characterization
of its dependence as a function of abundance. We find a value 0.2 dex lower
than solar metallicity at a galactocentric distance of 8.7 kpc, in agreement
with recent HII region studies using the weak forbidden auroral oxygen line at
4363 AE. We determine the effective stellar temperature, gravity, luminosity
and line-of-sight extinction of the blue supergiant being studied. We show that
it fits well on the flux-weighted gravity--luminosity relation (FGLR),
strengthening the potential of this method as a new extragalactic distance
indicator.
