The Galileo spacecraft was fortuitously situated for a direct view of the impacts of the fragments of comet Shoemaker-Levy 9 in Jupiter's atmosphere. The Galileo Near Infrared Mapping Spectrometer (NIMS) instrument observed several of the impact events in several discrete bands and with a temporal resolution of roughly five seconds. Data have been received for the G impact showing two phases of strong infrared emission. The first phase is approximately one minute in duration and corresponds to the initial fireball and early plume development. This is followed six minutes later by the onset of heating by plume ejecta falling back on the upper atmosphere. This report provides a preliminary description of the fireball phase. The first detection of the G fireball occurred at 07:33:37 UT on July 18, 1994, approximately five seconds after the initial signal recorded by the Galileo Photopolarimeter-Radiometer (PPR) instrument. The preceding NIMS measurement, occurring approximately one second before the initial PPR signal, showed no evidence of fireball emission. The detected duration of the fireball at 4.38 &mgr;m was 70 seconds. Spectra in the first half of this period show blackbody-like emission, with absorption features from overlying methane and molecular hydrogen. The strengths of these features place the fireball in the upper troposphere and lower stratosphere, above the ammonia cloud layer. The emitting surface rises and accelerates, achieving a velocity of 2--3 km/sec after 25 seconds, in qualitative agreement with that expected for an explosion in an inhomogeneous atmosphere. ¿ American Geophysical Union 1995