Middle-ultraviolet (210- to 252-nm) images have revealed the transpolar structure of polar mesospheric clouds (PMCs) at a spatial resolution of 3 km. The ultraviolet and visible imaging and spectrographic imaging instrument on the Midcourse Space Experiment (MSX) satellite collected over 27,000 mid-UV images of PMCs during 26 passes over the north and south polar regions during the summer seasons of 1997, 1998, and 1999. A Lomb periodogram analysis of PMC radiance projected to an 83-km altitude reveals periodic structures with wavelengths ranging from ~100 to ~3000 km. In either hemisphere, more PMCs have features with wavelengths shorter than 1000 km than longer, and a crude spectrum of the PMC structures suggests a spectral peak between 500 and 1000 km. There is little evidence of structures having wavelengths short of ~100 km, and the longer wavelengths generally have more spectral power than the shorter wavelengths. PMC structures do not remain stable over time periods of weeks, but may retain similar structural features for at least as long as 24 hours. The clouds may be considered markers of gravity waves, which carry energy from the lower atmosphere to the mesosphere and modulate the appearance of PMCs. ¿ 2000 American Geophysical Union