Large abundances of ClO were observed inside the arctic polar vortex during 14 flights of the NASA ER-2 aircraft from Stavanger, Norway (59¿N, 6¿E) to 80¿N latitude. Flights were conducted at altitudes between 14 and 20 km when the solar zenith angle was between 79¿ and 101¿. Data are reported for three flights, January 6, January 16, and February 10, that represent the main features observed during the mission. ClO mixing ratios were typically less than 50 parts per trillion by volume (pptv) outside the vortex and exceeded 100 pptv inside the vortex for all flights. ClO mixing ratios were more than 500 pptv for four days in a row in early February, reaching 1130 pptv on February 10, at an altitude of 19 km (potential temperature of 460 K). Peak ClO mixing ratios in early February were ~100 times larger than those observed at mid-latitudes for all altitudes surveyed between 15 and 19 km. These data, comparable to those obtained in the antarctic ozone hole, indicate that the springtime arctic polar vortex was extensively perturbed by heterogeneous chemistry and contained enough ClO to catalytically destroy ozone rapidly. ¿ American Geophysical Union 1990