  We present deep CCD photometry of a field in the newly discovered dwarf
spheroidal galaxy in Sagittarius (hereafter Sgr), and of a nearby control
field. These data (totaling ~3.5 hrs exposure time in V and I) were used to
produce color-magnitude (CM) diagrams reaching I ~ 22.3 in both fields. After
statistically removing the field-stars from the CM diagram we find that Sgr is
dominated by a moderately old (age ~ 10 Gyr) population, significantly younger
than a typical globular-cluster population. There is some evidence for a weak
intermediate-age component or alternatively a population of blue stragglers.
These results confirm that Sgr is a bona fide dwarf spheroidal (dSph) galaxy,
the ninth found orbiting the Milky Way. We have discovered nine short-period
variables in the Sgr field, well in excess of the number found in the control
field. Seven of these stars have similar apparent magnitudes and are almost
certainly RR~Lyr stars in Sgr. We have used the minimum-light colors of the
RRab variables to determine the reddening of the Sgr field to be E\vi\ = 0.22.
For an assumed RR~Lyr luminosity of M_V = +0.6 we conclude that the distance of
Sgr is 25.2 +/- 2.8 kpc. The properties of the Sgr giant branch and upper main
sequence are consistent with a mean metallicity of [Fe/H] ~ -1.1 +/- 0.3.
