  We report on the formation and evolution of a large-scale,
synchrotron-emitting jet from the black hole candidate and X-ray binary system
GX 339-4. In 2002 May, the source moved from a low/hard to a very high X-ray
state, contemporaneously exhibiting a very bright optically thin radio flare.
Further observations with the Australia Telescope Compact Array have tracked
the formation of a collimated structure extending to about 12 arcsec, with
apparent velocity greater than 0.9c. The luminosity of the outflow seems to be
rapidly decreasing; these observations confirm that transient large-scale jets
are likely to be common events triggered by X-ray state transitions in black
hole X-ray binaries.
