During the last year a new population of galaxies at high redshift
(z=2.5-3.5) has been identified which may represent the long
sought-after population of protogalaxies -- the progenitors
of normal galaxies like the Milky Way, observed in their
initial episode of star-formation.  We propose to use ISO to study
the spectral energy distribution of MS1512-cB58  at z=2.72, which is
by far the brightest example in its class -- 3 magnitudes brighter
than any other known high redshift protogalaxy candidate.  We will
combine ISOCAM LW2 and LW3 photometry, and LWS1 low resolution
spectroscopy from 43-197 microns, with a large body of ground-based
spectrophotometry and HST photometry to measure the current
star-formation rate, reddening, star-formation history, and
dust mass in cB58.  The observed mid-IR measured with ISOCAM
corresponds to the rest frame H and K band, and so will place
important constraints upon the presence of any old population of
stars.  The far-IR spectral energy distribution obtained with
the LWS will be combined with ground-based sub-millimeter
bolometry to estimate the temperature and mass of the dust in this
galaxy.