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===================================================================== ==> In this proposal, more time is being requested for CCESARSK.IDSPCO_1 to 7 ===================================================================== We propose to devote about 13 hours of ISO observation time to perform an important test for rival structure formation scenarios. This potentially decisive cosmological test will be achieved through an ultra-deep survey down to the ultimate performance limit of ISO, as of sensitivity, with one of its most sensitive photometric channels (CAM-LW3), over a sizeable sky area (7'x7'). After the advent of 10m-class telescopes and after refourbishment of the Hubble Space Telescope, optical investigations of the distant universe have made a decisive quantum jump, allowing to detect galaxies over a very wide redshift interval. All these discovered objects, however, are characterized by quite moderate Star-Formation rates. On the other hand, much more substantial SF events at high redshifts are called for by various independent arguments, including the large observed amounts of metals in hot intra-cluster and galactic plasmas, the observed dynamical and photometric properties of local early-type galaxies, the high D abundance at high-z, and the recently measured extragalactic far-IR background. Why optical observations have missed this enhanced SF event ? An obvious possibility is in the combined effect of high redshift and dust extinction, both shifting the emitted photons out of the optical band. An alternative solution could be a typically slow SF rate, but sustained over the whole Hubble time (somewhat in keeping with hierarchical formation scenarios), in which case no bright high-z phase would be expected. Though not competing with the spatial resolution and sensitivities reached from ground and by the HST in the optical-UV (e.g. in the HDF area), this proposed very sensitive ISO survey will allow to open a new spectral window in the deep sky, up to redshifts where we expect to test the existence of a bright, dust extinguished phase during the formation of spheroidal galaxies and quasars. Given the optimal combination of survey area, sensitivity and bandwidth,we judge this as a best opportunity for ISO to achieve a top-priority cosmological goal.