On June 14, 1995, the U.S. Navy's Relocatable Over-the-Horizon Radar (ROTHR) west of Corpus Christi, Texas, mapped the radial component of ocean surface current with 15-km resolution over a 230,000-km2 area in the Gulf of Mexico. Concurrently, an oceanographic research vessel measured near-surface currents within part of the area illuminated by the radar providing an opportunity to compare radar-derived surface currents with in situ sea truth. The R/V Gyre, operated by Texas A&M University, twice traversed the Gulf of Mexico Loop Current while measuring current vectors with an acoustic Doppler current profiler (ADCP). We compared radar-derived currents with currents measured in the uppermost ADCP bin (centered at 10-m depth). If only radar data exceeding a quality threshold are considered, the rms difference in the radial currents measured by the two techniques is 27 cm s-1. This difference most likely reflects the different sampling employed by these instruments, as well as unremoved ionospheric biases in the radar measurements. ¿ 1998 American Geophysical Union