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.