In July 1996, with the Hubble Space Telescope (HST), we observed the Pele plume silhouetted against Jupiter at a wavelength of 0.27 &mgr;m, the first definitive observation of an Io plume from Earth. The height, 420¿40 km, was greater than any plume observed by Voyager. The plume had significantly smaller optical depth at 0.34 and 0.41 &mgr;m, where it was not detected. The wavelength dependence of the optical depth can be matched by a plume either of fine dust, with minimum mass of 1.2¿109 g and maximum particle size of 0.08 &mgr;m, or of SO2 gas with a column density of 3.7¿1017 cm-2 and total mass of 1.1¿1011 g. Our models suggest that early Voyager imaging estimates of the minimum mass of the Loki plume [Collins, 1981> may have been too large by a factor of ~100. We may have detected the Pele plume in reflected sunlight, at 0.27 &mgr;m, in July 1995, but did not see it 21 hours earlier, so the plume may be capable of rapid changes.¿ 1997 American Geophysical Union