Fast response data taken aboard the National Oceanic and Atmospheric Administration WP-3D aircraft are used to determine the structure of atmospheric boundary layer turbulence on either side of a well-developed sea surface temperature front southwest of Bermuda. The data were taken on February 17, 1986, as part of the Frontal Air-Sea Interaction Experiment (FASINEX). A broad region of low-humidity air extending from 15 km to 35 km south of the front is probably due to the presence of a frontally induced secondary circulation. Evidence for a secondary flow is found in both the time series of atmospheric variables and the statistics obtained from conditionally sampled updrafts and downdrafts in the transect across the SST front. Larger sea-air temperature and humidity difference on the warm (south) side of the front give rise to surface layer sensible and latent heat and buoyancy fluxes that are larger than those on the cold side. Turbulence structure appears to be influenced as much by the presence of strong wind shear at the top of the boundary layer as by differing conditions at the surface on either side of the front. A larger rate of entrainment on the warm side of the front is indicated by the greater influence of low-momentum air from the overlying shear layer on updrafts in the upper part of the mixed layer, as well as the more frequent overtunning of cool/moist updrafts and warm/dry downdrafts. It is conjectured that the larger entrainment rate is due to the interaction between the inversion layer and more energetic updrafts produced by greater surface forcing on the warm side of the front. ¿ American Geophysical Union 1989