  Isotopic abundance ratios 24^Mg:25^Mg:26^Mg are derived for 20 stars from
high- resolution spectra of the MgH A-X 0-0 band at 5140AA. With the exception
of the weak g-band giant HR 1299, the stars are dwarfs that sample the
metallicity range -1.8 < [Fe/H] <0.0. The abundance of 25^Mg amd 26^Mg relative
to the dominant isotope 24^Mg decreases with decreasing [Fe/H] in fair accord
with predictions from a recent model of galactic chemical evolution in which
the Mg isotopes are synthesised by massive stars. Several stars appear
especially enriched in the heavier Mg isotopes suggesting contamination by
material from the envelopes of intermediate-mass AGB stars.
