  Based on a power spectrum analysis of the IRAS ISSA maps, we present the
first detection of the Cosmic far-Infrared Background (CIB) fluctuations at 60
and 100 microns. The power spectrum of 12 low cirrus emission regions is
characterized by a power excess at spatial frequencies higher than k~0.02
arcmin^{-1}. Most of this excess is due to noise and to nearby point sources
with a flux stronger than 1 Jy. But we show that when these contributions are
carefully removed, there is still a power excess that is the signature of the
CIB fluctuations. The power spectrum of the CIB at 60 and 100 microns is
compatible with a Poissonian distribution, at spatial frequencies between 0.025
and 0.2 arcmin^{-1}. The fluctuation level is ~1.6x10^3 Jy^2/sr and ~5.8x10^3
Jy^2/sr at 60 and 100 microns respectively. The levels of the fluctuations are
used in a larger framework, with other observationnal data, to constrain the
evolution of IR galaxies (Lagache et al. 2002). The detections reported here,
coupled with the level of the fluctuations at 170 microns, give strong
constraints on the evolution of the IR luminosity function. The combined
results at 60, 100 and 170 microns for the CIB and its fluctuations allows, on
the CIB at 60 microns, to put a firm upper limit of 0.27 MJy/sr and to give an
estimate of 0.18 MJy/sr.
