Thirty-six balloon flights, carrying various particle counters, are used to study the stratospheric aerosol layer during the year following the Mt. St. Helens volcanic eruption which occurred on May 18, 1980. Aerosol enhancements were observed to altitudes as high as 26 km. For the first time, stratospheric condensation nuclei (r≥0.01 μm) were observed in conjunction with a volcanic eruption. Time variation of these small particles suggests their continuous nucleation for at least 1 1/2 months after the eruption. Particles formed in the stratosphere following this eruption were generally smaller than those observed following the eruption of Fuego in 1974. This may be related to a relative excess in condensation sites at the time of the Mt. St. Helens eruption, thus resulting in smaller particles due to increased competition for the available sulfurous vapors involved in the nucleation process. The excess mass concentration of volatile material which remained in the stratosphere about 6 months after the eruption was of the order 10-7 g cm-2 (column) and if assumed to be uniform over the northern hemisphere, would amount to about 0.25 Tg. This is about 3 times the background level but only one half that injected by the eruption of Fuego in 1974.