Anticyclonic eddy X91 was observed (by remote sensing) from May 1991 to February 1992 in the southern Bay of Biscay. Two drogued Argos buoys deployed in the center in September showed that particle separation rates in the core of the eddy at a depth of 100 m were of the order of 100 m/day. Similar values were found for the mean radial velocity component of the buoys with respect to the eddy center. The center of the eddy core (~10-km radius) was in near-solid-body rotation with a period approaching 3.5 days and relative vorticity of -0.4f. Deep expendable bathythermograph profiles (to 2000 m) in September gave an eddy diameter of about 100 km with geostrophic velocities falling to half values at a depth of 800 m. The geostrophic azimuthal transport was ~10 Sv; the kinetic energy was ~1.4¿1014 J. X91 was resurveyed in December, and the azimuthal currents extended to greater depths (~1 km). The transport was still ~10 Sv but the kinetic energy was reduced to ~75% of the September value. Drogued buoys showed a slower (~10%) rotation period in December. X91 is a good example of a coherent vortex structure, and the structural similarity between X91 and F90a (the slope water oceanic eddy observed in 1990) suggests that X91 is also a swoddy. ¿ American Geophysical Union 1992