A sunlit cloud was observed near the horizon at South Pole Station (90 ¿S), four months after summer solstice in 1992, at a solar depression angle of 14.6¿. The angular location of the transition from sunlit to twilit cloud in the photograph establishes the cloud height at about 80 km. The cloud extended horizontally at least from 81¿ to 85 ¿S and from 40 ¿W to 20 ¿E. The probable origin of this cloud by formation of water-ice crystals near the mesopause suggests that mesospheric temperatures occasionally deviate by at least 70 K from their climatological means. An alternative explanation that the cloud was a debris cloud formed by disintegration of a reentering man-made satellite is ruled out. A debris cloud from an entering meteoroid is a possible explanation but would be unprecedented. Hydroxyl airglow emissions at the south pole in May give a frequency of 1% for mesopause temperatures below 155 K, consistent with the rarity of out-of-season sightings of noctilucent clouds in Antarctica.¿ 1997 American Geophysical Union