Little is known about the variability of Venus' upper atmosphere. We report discovery of a large (∼ ±30–50%) 9-day period density oscillation, derived from radar tracking of the Magellan spacecraft from 15 Sept 92 – 24 May 93. The densities correspond to 164–184 km altitude, 11°N latitude, and cover all local times. The wave is presumed to propagate upward from lower atmosphere regions, and occurs mainly between dusk and midnight, suggesting that local time differences in forcing and/or mean wind filtering are affecting accessibility to the upper atmosphere. Possible sources for the wave are discussed, including wave-wave interactions. Many questions remain concerning the origin and nature of the 9-day oscillation, including its persistence. Instruments on Venus Express, now orbiting Venus, have the opportunity to further elucidate the 9-day wave and understand its role in the dynamics of Venus' upper atmosphere.