We have measured stratospheric chlorine monoxide (ClO) at Ny ¿lesund (79 ¿N), on the Arctic island of Spitzbergen, during the spring of 1997 as part of a ground-based millimeter-wave radiometer intercomparison campaign. The presence of ClO in the stratosphere is a direct result of the catalytic destruction of ozone by chlorine and can provide a good measure of this process. Spitzbergen remained well inside the strong, cold polar spring vortex of 1997 during most of our observation period, which ran from February 7 through March 17. Measurements show a strong enhancement of lower stratospheric ClO from mid-February through mid-March, in substantial agreement with other measurements, and consistent with increasing daily exposure to photolytic processes liberating active chlorine. A comparison of these results with ClO observations made at McMurdo Station (78 ¿S), Antarctica, during the austral spring of 1997 shows ClO amounts in the Arctic stratosphere that were comparable to Antarctic stratospheric values when temperatures and solar exposures were similar over a relatively short time. ¿ 1999 American Geophysical Union