Ultraluminous infrared galaxies have been known for more than a decade, but the source of their very large far-infrared luminosities remains controversial. It may reflect a quasar-like active nucleus surrounded by a torus of dense gas and dust, the latter absorbing the energetic photons from the nuclear region and re-emitting at infrared wavelengths’, or a huge burst of massive-star formation in dense dusty clouds of molecular gas close to the nucleus(2), which heats the surrounding dust. A number of ultraluminous galaxies are also a source of OH-megamaser emissions (intense laser-like spectral lines at microwave frequencies), an observation that may hold important clues as to the main power source in these galaxies. A general feature of many models(3,4) is that the masers are pumped radiatively by the absorption of infrared photons. Identifying the source of the maser pump may therefore indicate whether the ultimate energy source is a burst of star formation, or an active nucleus. Here we report the detection of a strong midinfrared OH absorption line in the prototypical megamaser and ultraluminous galaxy(5), Arp220. We find that the power absorbed in this line alone is sufficient to pump the megamaser emission seen at radio wavelengths. Moreover, the warm, extended nature of the pumping region is suggestive of a starburst origin for the ultraluminous infrared emissions.