We have detected SO2 in ultraviolet spectra of Callisto obtained with the Hubble Space Telescope's Faint Object Spectrograph. An absorption band centered at 280 nm appears in the spectrum of Callisto's leading hemisphere, but is not apparent in the spectrum of the trailing hemisphere. The band is similar to the SO2 band on Europa's trailing hemisphere. Callisto's leading hemisphere spectrum can be well fit with models that include SO2 ice absorption with N(SO2)≥6¿1016 cm-2. Callisto's leading hemisphere is modified by impacts with micrometeorites; this may directly or indirectly be a source of sulfur dioxide.¿ 1997 American Geophysical Union