We present CCD BVI photometry of the old open cluster Berkeley 21, one
of the most distant clusters in the Galactic anticentre direction, an
d possibly the lowest metallicity object in the open clusters sample.
Its position and metal abundance make it very important for the study
of the Galactic disc. Using the synthetic colour-magnitude diagram met
hod, we estimate values for the distance modulus (m - M)(0) = 13.4-13.
6, reddening E(B - V) = 0.74-0.78 (with possible differential absorpti
on), and age = 2.2-2.5 Gyr.