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Based on simple starburst models of a limited sample of ISO LWS
observations of four ultraluminous infrared galaxies (L_IR > 10^12
L_solar;  ULIGs), we have found that the age of the starburst responsible
for the IR luminosity of the galaxy might be measured by it's [CII] 158 um
/ FIR ratio.  We propose a new set of observations of a sample of 11 ULIGs
at redshifts < 0.15 to help confirm and extend this conclusion, provide an
evolutionary sequence, and at the same time test the suggestion that
starbursts (not AGNs) power all of these ULIGs (Lutz et al. 1996).  As
local analogs of high-z protogalaxies, the new ULIG observations will also
test the usefulness of [CII] as a tracer of high-z objects and,
ultimately, as a diagnostic of galaxy formation models.  The assembled
team consists of experienced ISO observers from a broad cross-section of
institutions.