Modeling and observations provide evidence of the existence of a tertiary ozone maximum in the middle mesosphere restricted to winter high-latitudes. This local maximum occurs at approximately 72 km altitude, at latitudes just equatorward of the polar night terminator. Model analysis indicates that this maximum is the result of a decrease in atomic oxygen losses by catalytic cycles involving the odd-hydrogen species OH and HO2. In the middle mesosphere, at high latitudes, the atmosphere becomes optically thick to ultra-violet radiation at wavelengths below 185 nm. Since photolysis of water vapor is the primary source of odd-hydrogen, reduced ultra-violet radiation results in less odd-hydrogen and consequently lower oxygen loss rates. The consequent increase in atomic oxygen results in higher ozone because atomic oxygen recombination remains the only significant source of ozone in the mesosphere. ¿ 2001 American Geophysical Union