Imaging of Io with the faint object and planetary cameras of the Hubble space telescope in 1992 and 1993 at wavelengths of ~3450, 3700, and 4100 ¿ shows two surface areas that have undergone significant, large-scale change in reflectivity since the 1979 Voyager encounters. The first is located in Colchis Regio and covers ~106 km2 between longitudes 150¿--180¿ and latitudes -25¿ to +30¿; the second is centered at longitude ~130¿, latitude ~+30¿, and extends for ~105 km2. Both areas have darkened by ≥45¿ since 1979. In light of the active volcanism discovered on Io by the Voyager 1 encounter, it seems reasonable to infer that these large-scale changes in surface morphology are due to some type of volcanic activity in the intervening 14 years. We hypothesize two possible causes for these darkenings; either large-scale eruptions of the Pele type have covered areas of existing SO2 surface frost with new, much darker deposits, or previously active eruptions of the Prometheus type (or smaller-scale venting), which apparently produce SO2 gas that is bright in the visible when it condenses as surface frost, have become inactive. ¿ American Geophysical Union 1995