ALTHOUGH objects that emit radio jets have been known for many years, the physical mechanism responsible for the jets has been unclear. Accretion of mass onto a compact object (such as a black hole) has often been invoked in models of their formation(1). X-ray emission from such sources is a potentially powerful probe of the processes taking place, because it seems to arise much closer to the central object. Here se report the detection of X-ray and radio emission from the recently discovered(2,3) transient source X-ray Nova Scorpii 1994 (GRO J1655-40). The radio outbursts, presumably reflecting the feeding of material into relativistic jets, generally follow bursts of hard X-ray emission (20-400 keV) with a delay that varies from a few days to about two weeks. This suggests that the mechanism behind the X-ray emission is not related to the ejection process in a simple way. Nevertheless, GRO J1655-40 may be the best example of a compact object/accretion-disk system in which models of jet formation and X-ray production can be tested directly.