We propose to search for galaxies at high (z~1) and ultra-high (z~5)
redshifts using ISOCAM at 15um.  In random fields the sensitivity of
ISO is sufficient only to see the most extreme objects at these redshifts,
which are very unlikely to be found in the small areas that can be searched.
We propose instead to use rich galaxy clusters as magnifiers of the
background field at high z to potentially increase the effective sensitivity
by 1-2 orders of magnitude.  With a magnification factor m=5-10, we will
be able to detect moderately luminous galaxies at z=1 in the rest frame
6-9um PAH band.  At z=5, we may be sensitive to the near-IR emission from
galaxies with IR luminosities substantially below those of the more luminous
objects in the local Universe.