We discuss the possibility that steplike structures are formed in subcritical regions of vertically stratified shear flow. The mechanism we propose essentially consists of localized intense mixing in highly stratified and sheared flows, probably following frontogenesis. Its main assumption is that the vertical density flux increases monotonically with decreasing gradient Richardson numbers, which corresponds to enhanced stratification and/or diapycnal shear. This differs from Phillips' <1972> mechanism, which we argue may not apply to vertically stratified shear flow. An essential condition for the formation of constant density steps is the incorporation of a Langevin type equation which takes into account that turbulence must last for some finite characteristic time. We present numerical computations for the case of approximately constant diapycnal shear which lead to the formation of a staircase depth-density structure. ¿ 1998 American Geophysical Union