Group velocity dispersion characteristics of fundamental mode Rayleigh waves (T = 20 to 100 sec) in the North and South Atlantic oceans have been determined from moving window analyses of seismograms. The “mixed path” velocity data combined with oceanic age information for the North and South Atlantic were inverted to yield “pure path” dispersion characteristics for four sea-floor age divisions in the North Atlantic (0 to 23 m.y., 23 to 63 m.y., 63 to 100 m.y., and older than 100 m.y.), for three sea-floor age divisions in the South Atlantic (0 to 23 m.y., 23 to 63 m.y., and older than 63 m.y.), and for one nonoceanic division. Rayleigh wave group velocities were found to increase with increasing oceanic age as has been previously described for the Pacific. The velocities in the Atlantic Ocean basins were found to be 5 to 8 per cent faster than those for waves of corresponding periods in the Pacific Ocean basins. The distinct differences in velocities between the Atlantic and Pacific oceans provide additional evidence that the upper mantles of these two oceans are not identical.