  We present the X-ray data and the optical identifications for a deep ROSAT
PSPC observation in the "Marano field". In the inner region of the ROSAT field
(15' radius) we detected 50 X-ray sources with Sx >= 3.7x10^(-15) erg/cm^2/s.
When corrected for the different sensitivity over the field, the estimated
observed surface density at Sx >= 4x10^(-15) erg/cm^2/s is 272+/-40
sources/sq.deg. Four X-ray sources, corresponding to 8% of the total sample,
have been detected in radio images with a flux limit of about 0.2 mJy. Careful
statistical analysis of multicolour CCD data in the error boxes of the 50 X-ray
sources has led to the identification of 42 sources, corresponding to 84% of
the X-ray sample. These 42 reliable identifications are 33 AGNs (including two
radio galaxies and one BL Lac candidate; 79% of the identified sources), 2
galaxies, 3 groups or clusters of galaxies and 4 stars. We also show that it is
likely that a few of the 8 unidentified sources are such because the derived
X-ray positions may be offset with respect to the real ones due to confusion
effects. The unidentified sources have a large ratio of X-ray to optical fluxes
and most of them have harder than average X-ray spectra. Since most of the
identified objects with these characteristics in our field and in the Lockman
field are AGNs, we conclude that also most of these sources are likely to be
AGNs. Finally, comparing the optically and X-ray selected samples of AGNs in
this field, we estimate the ``efficiency'' of AGN selection with deep X-ray
exposures and with purely optical criteria.
