The evolution of a submesoscale anticyclonic vortex was observed by high-frequency Doppler radio current meters and satellite radiometers. The vortex formed between two large cyclones to the southwest of Oahu, Hawaii. The radius of the core was ~15 km; the azimuthal velocity reached 35 cm s-1; and the surface vorticity remained below - f for 9 days, reaching an extremum of -1.7 f. The flow was ageostrophic near the center and around the periphery of the vortex. The initial growth may have been driven by negative wind stress curl in the lee of Oahu. The vortex was prone to inertial, symmetric, and anticyclonic ageostrophic instabilities, but the temporal evolution of radial profiles of vorticity was inconsistent with angular momentum redistribution by inertial instability. A tongue of surface water 0.7°C warmer became entrained northward between the vortex and the colder cyclone to the west. As the vortex strengthened, a 0.14°C km-1 front formed along the eastern flank of the tongue. The sea surface temperature gradient remained weaker on the western flank. The flow was anticyclonic (-0.4 f ) and divergent (0.1 f ) on the warm side of the front but cyclonic (0.6 f ) and convergent (-0.2 f ) on the cold side. This suggests ageostrophic cross-frontal circulations maintaining alongfront thermal wind balance in the presence of large-scale strain . Surface divergence d was proportional to vorticity ? during the 3-day frontogenesis: d ~ -(/ f ) ?. This is consistent with a semigeostrophic model of a front confined to a surface layer of zero potential vorticity.