Midlatitude stratospheric ClO at altitudes below 22 km has been measured for the first time. Measurements were made at latitudes between 27¿ N and 48¿ N during three flights from Moffett Field, California, in June and July of 1987, with a new instrument flown on the NASA ER-2 aircraft. The result from these flights is that the ClO mixing ratio increases from less than 0.5 pptv at 16.8 km to 2.0 pptv at 18.3 km and 10.1 pptv at 21 km. These altitude profiles agree with an extrapolated profile from our May 1986 balloon-borne experiment. ¿ American Geophysical Union 1988