  We have used the Marano1 field observations with ISOPHOT at 170 $\mu$m to
search for the Cosmic Far-InfraRed Background fluctuations. This field is part
of the FIRBACK project (Puget et al., 1999a). For the first time, fluctuations
due to unresolved extra-Galactic sources are isolated. The emission of such
sources clearly dominates, at arcminute scales, the background fluctuations in
the lowest Galactic emission regions.
  The study presented here is based on a power spectrum analysis which allow to
statistically separate each of the background components. With this analysis,
we clearly show that we detect the CFIRB fluctuations.
