Buoyancy length scale and internal disturbance Froude number criteria for turbulence collapse in stratified fluid are examined in the light of recent laboratory experiments, oceanographic observations, and numerical simulations. From these it emerges that the onset of collapse occurs when the turbulence integral length scale is of the order of the buoyancy length scale, giving a disturbance Froude number close to 1. Complete collapse of three-dimensional turbulence, characterized by nearly zero vertical mass flux, occurs at a lower value of the disturbance Froude number. In particular, a spatially inhomogeneous turbulence undergoes a drastic change in structure at a value of the disturbance Froude number of 0.2--0.3. In the collapsed state, internal wave motion and quasi two-dimensional turbulence coexist on different time scales, results supported also by the rotating turbulence analogy. Vertical diffusion coefficients and mixing efficiency, which are of direct practical importance, are also examined. ¿American Geophysical Union 1987