Latitudinal distributions of upper stratospheric ClO measured by MAS during the three ATLAS missions are presented for northern hemisphere (NH) spring equinox in 1992, southern hemisphere (SH) early fall in 1993, and NH fall in 1994. The MAS ClO results are shown along with correlative MLS observations. The results of both instruments consistently show the same latitudinal features. The ClO maximum in the NH spring occurs at mid latitudes, whereas the latitudinal ClO maximum in both the NH and SH fall occurs at high latitudes. The volume mixing ratio maxima were significantly higher in the fall (0.7--0.8 ppbv) than in spring (0.5--0.6 ppbv). Qualitatively, these results are consistent with calculations of several 2-D models. ¿ American Geophysical Union 1996