  We present deep $BVI$ observations of the dwarf irregular galaxy UKS1927-177
in Sagittarius. Statistically cleaned $V$, $(B-I)$ CMDs clearly display the key
evolutionary features in this galaxy. Previously detected C stars are located
in the CMDs and shown to be variable, thus confirming the presence of a
significant upper-AGB intermediate age population. A group of likely red
supergiants is also identified, whose magnitude and color is consistent with a
30 Myr old burst of star formation. The observed colors of both blue and red
stars in SagDIG are best explained by introducing a differential reddening
scenario in which internal dust extinction affects the star forming regions.
Adopting a low reddening for the red giants, $E(B-V) = 0.07 \pm 0.02$, gives
[Fe/H]=$-2.1 \pm 0.2$ for the mean stellar metallicity, a value consistent with
the [O/H] abundance measured in the HII regions. This revised metallicity,
which is in accord with the trend of metallicity against luminosity for dwarf
irregular galaxies, is indicative of a ``normal'', although metal-poor, dIrr
galaxy. A quantitative description is given of the spatial distribution of
stars in different age intervals, in comparison with the distribution of the
neutral hydrogen. We find that the youngest stars are located near the major
peaks of emission on the HI shell, whereas the red giants and intermediate-age
C stars define an extended halo or disk with scale length comparable to the
size of the hydrogen cloud. The relationship between the distribution of ISM
and star formation is briefly discussed.
