This proposal is prompted by the discovery of a far infrared isotropic
background in the COBE-FIRAS data (Puget et al 1995) which is likely to
be the extragalactic background due to early galaxies.  The background
and its structure are likely to be dominated by a population of high
redshift sources.  We propose a deep survey at 160 micron reaching
significantly below the confusion limit.  The extragalactic background
is an significant fraction of the total sky brightness at wavelengths
longer than 200 microns, and the proposed observations will extend the
ISO deep surveys to the longest wavelength accessible to ISO with good
sensitivity.  The proposed observation will give the statistical
properties of the fluctuations of the background and should be able to
detect the extragalactic component of these fluctuations if, as
predicted in some models, they dominate over the cirrus fluctuations in
the cleanest high latitude windows.