The mean sea surface relative to the International Union of Geodesy 1980 Geodetic Reference System reference ellipsoid has been derived from Seasat and GEOS 3 altimeter measurements. This surface, called MSS-9012, has been computed on a grid of 1/8¿ resolution. Each elevation value was calculated by fitting all data within 111 km to a local biquadratic surface using Bayesian least squares. Individual data points were weighted inversely to the square of the distance to the grid location in the gridding process. The surface covers the global ocean between 70¿N and 72¿S. In the vicinity of sea ice the altimeter heights have been corrected for the on-board tracker error that occurs over non-Gaussian surfaces. Comparisons are made between MSS-9012 and ocean bathymetry. The eastern extent of the Chain Fracture Zone in the Gulf of Guinea is more apparent in the altimetry than in the bathymetry data, as are many other features. The combination of data from the two satellites has successfully retrieved more information about the sea surface than was previously possible using only Seasat data. ¿ American Geophysical Union 1992