How did galaxies form and evolve is still a mystery to a large extent.
 Conflicting observational results and theoretical predictions did
 not allow yet to decide in favour either of the long-standing
 paradigm according to which spheroidal galaxies have ended their main
 formation phase just after one or two Gyrs after the Big-Bang, or of
 its current competiting scenario envisaging galaxy formation as a more
 gradual process of merging of smaller fragments taking place at recent
 cosmic epochs. A crucial observational result lies in the fact that
 current deep surveys in all e.m. bands have failed to reveal any
 populations of luminous starbursts corresponding to the main formation
 epoch. Because of its improvement in sensitivity with respect to
 previous surveys, there is a widespread feeling that ISO will
 substantially contribute to solve this enigma. This is based on the
 concept (see Franceschini et al., 1994, ApJ427,140) that our inability
 to detect forming galaxies might be due to the effect of extinction of
 the optical-UV emission by a dust-enriched ISM. If this is the case,
 the main energy output of primeval objects is expected to come out in
 the IR. Two approaches will be followed to explore this possibility.
 One is to survey blindly entire portions of the sky to the faintest
 possible flux levels. The alternative is to concentrate the efforts on
 a limited number of candidates discovered through direct (i.e. energy
 likage in the radio, mm, or near-IR) or indirect (e.g. gravitational
 lensing effects, spectral absorption in the direction of high-z QSOs)
 criteria. We propose to spend a few hours of the ISO lifetime to
 perform photometric imaging at both mid- and far-IR wavelengths on
 the currently best PG candidates (11 targets in Aut. and 12
 in Spring). This will contribute to provide not only a decisive test
 of current galaxy formation scenarios, but also invaluable information
 on some mysterious objects in the distant universe.