  We present a sample of synthetic massive stellar populations created using
the Starburst99 evolutionary synthesis code and new sets of stellar
evolutionary tracks, including one set that adopts a detailed treatment of
rotation. Using the outputs of the Starburst99 code, we compare the
populations' integrated properties, including ionizing radiation fields,
bolometric luminosities, and colors. With these comparisons we are able to
probe the specific effects of rotation on the properties of a stellar
population. We find that a population of rotating stars produces a much harder
ionizing radiation field and a higher bolometric luminosity, changes that are
primarily attributable to the effects of rotational mixing on the lifetimes,
luminosities, effective temperatures, and mass loss rates of massive stars. We
consider the implications of the profound effects that rotation can have on a
stellar population, and discuss the importance of refining stellar evolutionary
models for future work in the study of extragalactic, and particularly
high-redshift, stellar populations.
