Aerosol number concentrations were measured, using a photoelectric nucleus counter, at three locations in Antarctica. This nucleus counter was of high sensitivity and equipped with a diffusion battery to permit estimation of particle size. Concentrations of 50 to 100 cm−3 were predominant at the South Pole; concentrations of 100 to 150 cm−3 were most common at Siple Station (78°S, 84°W) where some maritime influence might be present. At both of these stations, concentrations of 500 to 1200 cm−3 very small particles were detected when the polar tropopause lowered. Concentrations of 50 to 150 cm−3 were found at Ross Island with continental winds; concentrations of 300 cm−3 accompanied maritime winds. Abstract Aerosol number concentrations were measured, using a photoelectric nucleus counter, at three locations in Antarctica. This nucleus counter was of high sensitivity and equipped with a diffusion battery to permit estimation of particle size. Concentrations of 50 to 100 cm−3 were predominant at the South Pole; concentrations of 100 to 150 cm−3 were most common at Siple Station (78°S, 84°W) where some maritime influence might be present. At both of these stations, concentrations of 500 to 1200 cm−3 very small particles were detected when the polar tropopause lowered. Concentrations of 50 to 150 cm−3 were found at Ross Island with continental winds; concentrations of 300 cm−3 accompanied maritime winds.