Comparison of a high resolution Viking image (422B10; 8 m/pixel) with a Mars Orbiter Camera image (SP2-502/06; 5.6 m/pixel) of dunes in the Acheron Fossae region of Mars (38 ¿N, 135 ¿W) reveals that the dunes moved <1 pixel during a span of almost 21 Earth years. Very shallow illumination in the MOC image indicates the dunes are <1.5 m high. The images indicate that any movement of these Martian dunes is <0.4 m/yr, a rate that is less than the documented movement of comparable dunes on Earth by a factor of up to 200. The Acheron Fossae dunes occur within a region of low thermal inertia, indicating that the dunes may be stabilized by a pervasive dust cover. Alternatively, the saltation threshold was not exceeded significantly at this location in more than 20 years. ¿ 2000 American Geophysical Union