Abstract Two satellites, Calsphere (1964-63C) and Dodecapole (1965-16G), have been used to determine weekly mean values of atmospheric density at heights near 1070 and 900 km respectively, between June 1968 and December 1970. Both satellites have nearly circular orbits and are therefore ideal for studying long-term variations in the atmosphere. After correcting the values of density to both a fixed height and a fixed level of solar activity, it is possible to trace for the first time the progress of the semi-annual variation in the exosphere over 2 1 2 years of high solar activity. The semi-annual variation in density is large, with the average value for the ratio of the October maxima to the July minima being about 2. When the results are combined with earlier data from Echo 2 there is evidence of a strong oscillation in the amplitude with a periodicity of 3 years.