T Tauri stars are young stellar objects, similar in mass to the Sun, that are completing the transition between a collapsing cloud and a main-sequence star powered by hydrogen fusion(1). Many of these objects are associated with circumstellar (and, presumably, protoplanetary) disks(2), as well as energetic outflows of gas that can extend several light years away from the young star(3), These outflows are thought to be collimated by magnetic fields(4), but direct observational evidence for such fields has hitherto been lacking, Here we show that the infrared companion(5) (T Tau S) of the prototypical T Tauri star (T Tau itself) recently ejected in opposite directions two large lobes of mildly relativistic particles, The radio emission from the two lobes exhibits strong circular polarization of opposite helicity, implying the existence of a strong, ordered magnetic field at a surprisingly large distance from the source, We also find that the T Tau system may contain three stars, rather than two as previously thought.