An experimental study is described that deals with the nature of the entrainment zone of a linearly stratified fluid when the turbulence in the mixed layer is generated by an oscillating grid. Measurements include the interfacial‐layer thickness, frequency, and amplitude of the interfacial waves and the buoyancy flux at the entrainment interface. These measurements were compared with a theory proposed by Long [J. Fluid Mech.84, 43 (1978)], part of which (the mixed‐layer deepening law) has already been verified by Folseetal. [Phys. Fluids24, 396 (1981)]. After a certain depth of the mixed layer, the entrainment rate shows an abrupt increase, and this may possibly be caused by wall effects as shown in our previous investigations.