An absorption band at 260 nanometers on the trailing hemisphere of Ganymede, identified as the Hartley band of ozone (O-3), was measured with the Hubble Space Telescope, The column abundance of ozone, 4.5 x 10(16) per square centimeter, can be produced by ion impacts or by photochemical equilibrium with previously detected molecular oxygen (O-2). An estimated number density ratio of [O-3]/[O-2] approximate to 10(-4) to 10(-3) requires an atmospheric density orders of magnitude higher than upper limits from spacecraft occultation experiment’s. Apparently, this O-2-O-3 "atmosphere" is trapped in Ganymede’s surface ice, an inference consistent with the shift and broadening of the band compared with the gas-phase O-3 band.