To estimate how well ERS-1 data from the 35-day repeat mission can resolve fine structure in the marine geoid or gravity field, we compare along-track gravity and gridded gravity fields derived from ERS-1 fast delivery altimeter data to those computed from GEOSAT Geodetic Mission altimeter observations in the Southern Ocean. We find that single passes of ERS-1 data can resolve along-track gravity anomalies with wavelengths as short as 28--30 km (i.e. comparable to Geosat). However, the ERS-1 gridded gravity field can only resolve anomalies as short as 66 km (versus ~18 km for Geosat). This two-dimensional resolution is limited by the larger ground track spacing (~35 km at 60¿S) of ERS-1. To resolve fine structure in the marine gravity field it is essential to have altimeter data along more closely-spaced ground tracks, such as those planned for the ERS-1 176-day repeat mission in 1994.