A medium frequency interferometer (MFI) has been established in Sondrestrom, Greenland, to measure the direction of arrival (DOA) of auroral roar emissions at 2.5–3.0 MHz. Combined with electron density data from the Sondrestrom incoherent scatter radar (ISR), these DOA data provide a powerful test of the most-cited generation mechanism of auroral roar emissions. This mechanism requires that the relationship between the electron plasma frequency (fpe) and the electron gryofrequency (fce) in the source regions isfpe2 = (n2 − 1)fce2, where n = 2, 3 is the harmonic number of the emission. For each of five auroral roar events studied, to within experimental uncertainty this frequency-matching condition is met throughout the source region of the emissions inferred from the MFI and ISR data. Furthermore, ray-tracing analysis indicates that signals at the roar frequency are able to propagate from the source region to the location of the MFI receiver.