  We report on $UBVRI$ CCD photometry of two overlapping fields in the region
of the intermediate-age open cluster NGC 2158 down to V=21. By analyzing
Colour-Colour (CC) and Colour-Magnitude Diagrams (CMD) we infer a reddening
$E_{B-V}= 0.55\pm0.10$, a distance of $3600 \pm 400$ pc, and an age of about 2
Gyr. Synthetic CMDs performed with these parameters (but fixing $E_{B-V}=0.60$
and $[{\rm Fe/H}]=-0.60$), and including binaries, field contamination, and
photometric errors, allow a good description of the observed CMD. The elongated
shape of the clump of red giants in the CMD is interpreted as resulting from a
differential reddening of about $\Delta E_{B-V}=0.06$ across the cluster, in
the direction perpendicular to the Galactic plane. NGC 2158 turns out to be an
intermediate-age open cluster with an anomalously low metal content. The
combination of these parameters together with the analysis of the cluster
orbit, suggests that the cluster belongs to the old thin disk population.
