Geosat Exact Repeat Mission (ERM) altimetric measurements of the sea surface height in the Gulf of Mexico are used to determine the mean sea surface height with respect to the ellipsoid and mesoscale variability along Geosat ground tracks in the gulf for the time period from November 8, 1986, to November 25, 1988. The alongtrack mean sea surface is determined using a regional crossover adjustment procedure, in which the tilt and bias of the mean arcs are estimated using a least squares technique to minimize the height differences at crossovers points. A mean surface generated using the Geosat ERM alongtrack mean is calculated and contrasted with a previously derived mean surface determined using GEOS 3 and Seasat crossover differences. This provides a first look at the variability in the mean between the time periods of 1987--1988 and 1975--1978. In addition, the alongtrack mesoscale variability time series has been produced from the Geosat ERM data set by using a robust orbit error removal algorithm to determine the variability of the sea surface height with respect to the alongtrack mean. A surface generated using the rms of this alongtrack time series shows good qualitative and quantitative agreement with previous in situ observations in the region. This study demonstrates the potential of satellite altimetry for oceanographic studies of the Gulf of Mexico. ¿ American Geophysical Union 1990