Observations of temperature and salinity fine structure near the edge of a newly formed Gulf Stream warm core ring are presented. A thermohaline front and a cold tongue of shelf water were tracked for about 16 km in a series of crossings by a depth-cycling CTD. The core was observed to become warmer saltier, and more dense in the downstream direction, leading to estimates of eddy diffusivities larger than 10-3 m2/s. It is concluded that fluxes this large cannot be due to double-diffusive processes, either lateral or vertical. Shear-driven turbulence can account satisfactorily for the mixing of the core, but it would require that the Richardson number be near 1/4, which is much smaller than that estimated from thermal wind.