We propose to observe a specially selected high redshift starburst
 galaxy, IRAS F15307+3252 at z = 0.93, in order to probe the physical
 conditions deep within the source.  This object is one of the most
 luminous in the universe, and may be going through its first phase of
 star formation. The source is abundant in molecular gas and dust
 because its rest frame emission shows the 100-200 micron peak
 indicative of warm (100K) dust.  This dust, however, shields the
 nucleus from all but mid-IR and longer wavelengths.  By observing
 these sources in several carefully selected mid-IR diagnostic emission
 lines, we expect to positively identify the nature of the central
 heating source, that is to distinguish between starburst and AGN power
 sources.  The ISO satellite is ideally and uniquely suited to this
 project, as it will be able to detect dust penetrating mid-IR
 diagnostic emision lines which cannot be seen from the ground.