I consider some plasma effects to be expected when the fragments of Comet Shoemaker--Levy 9 crash into Jupiter. Emphasis is put on pre-impact effects, and on the possibility of significant radio emission. It is shown that an ionizing shock will develop at altitudes below about 600, km and that significant ionization will exist above that altitude. The incoming comet and its surroundings will then represent a rapidly moving conductor in a strong magnetic field and will induce large currents in Jupiter's magnetosphere, similar both qualitatively and quantitatively to a way in which Io interacts with Jupiter's magnetic field and with the plasma of the torus. This process is thought to be involved in the generation of decametric radiation, so that the impact of the comet may also produce such radiation. The power involved in this interaction is estimated to be at least 1/15th of the power due to Io, and its duration to be for 10--20 s before the comet's impact with the surface of Jupiter. ¿ American Geophysical Union 1994