We use historical data <Bane et al., 1981>, interpolated to isopycnic coordinates, to examine the possibility of significant diapycnal mixing within the upper thermocline layers of the Gulf Stream. The data consist of 28 air-dropped expendable bathythermograph (AXBT) sections in the northern Blake Plateau distributed in five different surveys done within 8 days. From the data we obtain a separation index between isopycnals j=&rgr;∂z/∂&rgr;, where z is the depth of an isopycnal and &rgr; is the density; the diapycnal shear ∂v/∂&rgr;, where v is the geostrophic velocity; and the gradient Richardson number Ri. Following Pelegr¿ and Csanady <1994>, we postulate that the material derivative of the density, or density tendency w&rgr;=D&rgr;/Dt, is the result of small-scale instabilities related to near-critical Ri values. We present the distribution of these quantities (j, ∂v/∂&rgr;, Ri, and w&rgr;) and the diapycnal convergence/divergence (∂w&rgr;/∂&rgr;) over isopycnals and on sections normal to the coast. The results show the passage of steep meanders being related to anomalously low j values (strong density gradients) within the upper thermocline layers and the cyclonic filaments of the stream. The statically stable upper thermocline layers, however, are concurrent with large diapycnal shear and turn out to be dynamically unstable, characterized by low Ri and high-density tendencies and diapycnal convergence/divergence. The errors involved in calculating the dependent variables from AXBT data are assessed using both an error propagation approach and a Monte Carlo error simulation. These errors, although significant, are not large enough to modify the observed patterns substantially. ¿ 1999 American Geophysical Union