LDA measurements, together with torque measurements, have been carried out in agitated vinyl polymer solutions possessing yield stresses to clarify the formation and growth of a cavern, outside which the fluid is essentially stagnant due to the yield stress. At lower impeller speed, cavern growth is almost the same in baffled and non-baffled vessels. Under the condition where the cavern has grown up to the side wall of the vessel, considerable differences have been observed; in the non-baffled vessel the cavern height increases continuously with impeller speed, but in the baffled vessel it does not increase until the axial flow accumulates the energy to expand the cavern vertically again. Baffled mixing requires more power than non-baffled mixing to attain the fully agitated condition, A model is developed under the assumption that the fluid motion ceases if the shear stress in a dissipation zone is smaller than the fluid yield stress. The cavern diameter and height tall be related to the baffled mixing condition by this model, In addition, a correlation equation is obtained for the cavern height in the non-baffled vessel, to which the model is not applicable.