During the Skylab 4 mission the S-193 radar altimeter was operated nearly continuously for a revolution around the world on January 31, 1974. This direct measurement to the sea surface has provided for the first time an independent basis for the evaluation of the precision of global geoids computed from satellite-derived earth gravity models. This paper presents comparisons between the Skylab data and several recent gravity models published by Goddard Space Flight Center, the Smithsonian Astrophysical Observatory, and the National Oceanic and Atmospheric Administration. The differences between the altimeter geoid and the satellite geoids were as large as 20 m, rms values ranging from 8 to 10 m. These differences also indicated a systematic long wavelength variation (~100¿) not related to error in the Skylab orbits. Truncation of the models to degree and order 8 did not not climinate the long wavelength variation, but in every case the rms agreement between the satellite geoids and the altimeter geoid was slightly improved. Orbits computed with the truncated models were in contrast found to be inferior to those computed with the complete models.