  Far-infrared maps obtained with ISOPHOT have been searched for point-like
sources. The majority of the 55 sources is believed to be extragalactic and in
most cases no previously known sources can be associated with them. Based on
the far-infrared spectral energy distributions it is likely that
dust-enshrouded, distant galaxies form a significant fraction of the sources.
  We estimate the number densities of extragalactic sources at 90, 150 and 180
um wavelengths and at flux density levels down to 100 mJy. The counts are
compared with models of galaxy evolution. The counts exceed the predictions of
current models, even those with strong evolution, and no-evolution models are
rejected at a high confidence level.
  Comparison with recent results from COBE mission indicates that at 90 um the
detected sources correspond to over 20% of the extragalactic background light.
At longer wavelengths the corresponding fraction is 10%.

Astrophysics