  We discuss the rationale and the present status of a large redshift survey
aiming at measuring distances for all the clusters of galaxies detected by the
ROSAT X--ray All--Sky Survey in the southern hemisphere, with flux larger than
$\sim 2\times 10^{-12}$ erg s$^{-1}$ cm$^{-2}$. The survey is being performed
using three ESO telescopes in parallel. The final sample will contain about 700
clusters. It will represent a unique database for the study of the spatial
distribution of clusters, their X--ray luminosity function and their intrinsic
physical properties. Serendipitous results obtained so far include two new
gravitational arc systems in two clusters of galaxies. These findings, obtained
on rather short exposures, confirm the advantage of searching for arcs in
X--ray selected clusters.
