  Using the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope,
we have obtained rest frame mid-infrared spectroscopy of two bright
submillimeter galaxies. SMMJ02399-0136 at z=2.81 shows a superposition of PAH
emission features and a mid-infrared continuum, indicating significant and
roughly equal contributions to its bolometric luminosity from star formation
and from a Compton-thick AGN. We derive a new redshift of z=2.80 for
MMJ154127+6616 from the IRS spectrum and find this object is dominated by
starburst PAH emission. The rest frame mid- to far-infrared spectral energy
distributions are consistent with these submillimeter galaxies being scaled up
versions of local ultraluminous infrared galaxies. The mid-infrared spectra
support the scenario that submillimeter galaxies are sites of extreme star
formation and represent a key phase in the formation of massive galaxies.
