We have examined measurements of chlorine monoxide, ClO, in the lower stratosphere by the Microwave Limb Sounder (MLS) on the Aura satellite in the austral spring of 2005. These measurements have been compared to those of a ground-based spectrometer at Scott Base, Antarctica. The data analysis is performed in both cases by subtracting nighttime measurements from daytime ones. The transition from full darkness to full daylight at the high latitude of Scott Base limits the time during which both day and night measurements are made. After further selection for good observing conditions and the position of the polar vortex, 16 valid profile comparisons are made. The day-to-day variability of ClO is observed to be large, ∼30% of its peak value. The daily column densities of the two instruments are correlated with a significance of 3σ with most of the mean difference arising from 2 days. The statistical agreement between MLS and the Scott Base instrument is good. Scott Base values are on average marginally but not significantly larger, by 0.10 ± 0.07 ppb, or 11 ± 8% (1σ), in peak mixing ratio, than the MLS values.