We have made new measurements of stratospheric chlorine monoxide (ClO) at McMurdo Station, Antarctica, during the austral spring of 1987. Rotational emission line spectroscopy, employing a ground-based detector, was used to determine mixing ratio profiles over the range ~17--45 km. A spectral band pass double that was used for similar measurements in 1986 allowed an improvement to be made in the definition of the anomalous low-altitude stratospheric ClO layer associated with springtime ozone depletion. A peak minimum ratio of 1.6¿0.4 parts per billion by volume (ppbv) (95% confidence level) was found at 19.5¿1 km at midday during the period September 20--24, 1987. The observed peak mixing ratio and diurnal behavior are discussed in relation to chemical depletion theories. Calculations indicate that the large observed ClO concentration provides an efficient closure for a catalytic Cl cycle through the ClO dimer mechanism, yielding good agreement with various observed features of O3 depletion. ¿ American Geophysical Union 1989