The apparent diapycnal diffusivity below the wind-driven surface mixed layer of the ocean was determined in an anticyclonic eddy in the eastern North Atlantic using sulfur hexafluoride (SF6) tracer data collected in June 1998. In this tracer experiment the downward penetration of SF6 was measured for 3 weeks following the deliberate injection of SF6 in the surface mixed layer. The resulting data were used to constrain the one-dimensional Fickian diffusion model to estimate the diapycnal diffusivity. The model also includes the lateral diffusion component so that it can more accurately represent the time evolution of the SF6 concentrations along the isopycnal surface. This affects the estimation of the diapycnal diffusivity. For the upper thermocline immediately below the surface mixed layer we estimated the diapycnal diffusivity for the 3 week period as 0.3 ¿ 0.2 cm2 s-1 at a buoyancy frequency of 8.2 cph.