We describe the results from a laboratory experiment designed to directly measure the stress exerted across a sheared interface which lies between two convecting layers. The two-layer convection was set up in an annular tank which was rotating slowly so as to provide a basic ''store'' of angular momentum. Slow radial pumping was used in each layer to drive azimuthal flows, and measurement of the layer velocities provided near-direct measurements of azimuthal stress. The stress was found to be consistent at all times with that due to molecular viscosity and was independent of the buoyancy flux associated with the convection. The uncertainties of the experimental observations, taken together with the hypothesized stress-shear relation of Ruddick (1985; equation (1) of this paper), imply a maximum turbulent Prandtl number (absolute value of the ratio of momentum and density diffusivities) of O(1). The experimental results of Ruddick (1985) are reconsidered, and it was concluded that the dependence of internal wave decay rate of buoyancy flux observed in those experiments is a result of the large eddy viscosity in the turbulent mixed layers acting on the strain field of the wave. ¿ American Geophysical Union 1989