  FIRBACK is a deep survey conducted with the ISOPHOT instrument aboard the
Infrared Space Observatory (ISO) at an effective wavelength of 175 $\mu$m. We
present here results we have obtained on the first field, the so-called Marano1
which covers around 0.25 square degree. We find that the source density for
objects with a flux above 200 mJy exceeds the counts expected for sources found
in the IRAS deep surveys with a similar flux by about an order of magnitude.
Such an excess was expected on the basis of the high far infrared background
detected with the FIRAS and DIRBE instruments aboard the Cosmic Background
Explorer (COBE). These sources are likely to be redshifted infrared galaxies.
The steep number counts indicate strong cosmological evolution in this
population. The detected sources account for only 10 % of the cosmic IR
background. An extrapolation of the counts down to about 10 mJy would be
needeed to account for the whole background at this wavelength.
