  We present new ISO-SWS data for a sample of 27 starburst galaxies, and with
these data examine the issues of formation and evolution of the most massive
stars in starburst galaxies. Using starburst models which incorporate time
evolution, new stellar atmosphere models for massive stars, and a starburst
model geometry derived from observations of the prototypical starburst M82, we
model the integrated mid-infrared line ratio [NeIII](15.6 microns)/[NeII](12.8
microns). This line ratio is sensitive to the hardness of the stellar energy
distribution and therefore to the most massive stars present.
  We conclude from our models, with consideration of recent determinations of
the stellar census in local, high-mass star forming regions, that the
[NeIII]/[NeII] ratios we measure are consistent with the formation of massive
(~50-100 solar mass) stars in most starbursts. In this framework, the low
nebular excitation inferred from the measured line ratios can be attributed to
aging effects. By including estimates of the ratio of infrared-to-Lyman
continuum luminosity for the galaxies in our sample, we further find that most
starbursts are relatively short-lived (1-10 million years), only a few O-star
lifetimes. We discuss a possible cause of such short events: the effectiveness
of stellar winds and supernovae in destroying the starburst environment.
