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 ± 0.10,
a distance of 3600 ± 400 pc, and an age
of about 2 Gyr. Synthetic CMDs performed with these parameters
(but fixing E(B-V) = 0.60 and [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.