Solar prominences are sheets of relatively cool and dense gas embedded in the surrounding hotter corona. An erupting prominence can inject a mass of up to 10(15) g into the solar wind(1) as part of a coronal mass ejection. These eruptions must depend critically on the topology of the prominence＇s magnetic field. In all present models(2,3), the prominence hangs on horizontal or helical field lines, while an overlying magnetic arcade temporarily restrains the prominence from erupting. Such models are inconsistent, however, with the slow upward vertical gas flows that are seen in prominences-(4,14). Here we report counter-streaming flows along closely spaced vertical regions of a prominence, between its top and the lower solar atmosphere. As the flows must be aligned with the magnetic field, this observation implies that a field connects the prominence directly to the photosphere, contrary to all existing models. These magnetic ＇tethers＇ might help prevent a prominence from erupting.