Theoretical expressions are derived in a simple, but approximate, way for variations of vertical diffusivity and overturning length with Froude number, gradient Richardson number, and Reynolds number, in both sheared and unsheared stratified flow. These variations are then compared with observations in laboratories and oceans. For the case of strongly stratified tubulence (Froude number near unity) a previous value of 0.8 for mixing efficiency is attributed to an incorrect assumption for the numerical value of Lm/LB, the ratio of overturning to buoyancy length, and the efficiency is recalculated by use of recent measurements of that length ratio. ¿ American Geophysical Union 1992