The STRATEOLE project, organized by France's Centre National de la Recherche Scientifique (CNRS), will release a large number of isopycnal balloons to drift in the lower stratosphere of the southern hemisphere. In preparation for STRATEOLE this paper studies the structure and dispersion properties of the flow in the lower stratosphere at high southern latitudes during winter and spring. The approach for investigation is based on computing trajectories of fluid parcels and isopycnal balloons using the velocity field obtained on-line during simulations with a three-dimensional primitive-equation model of the stratosphere and mesosphere. A scheme for computation of isopycnal balloon trajectories is devised, and a method for estimating the location of the polar night vortex edge is developed. It is found that trajectories of fluid parcels initiated well inside the vortex remain within the vortex for a period of months. A small number of fluid parcels released near the vortex edge cross this edge with about an equal number of crossings from inside to outside the vortex as from outside to inside. The isopycnal balloons show a stronger tendency to cross the vortex edge than do fluid parcels. The relative dispersion properties of the flow in the lower stratosphere are analyzed in the light of theories on two-dimensional turbulence. It is determined that the flow inside the polar vortex can be considered as quasi two-dimensional, isotropic, homogeneous, and stationary for a period of about 6 days. Outside the vortex the differential advection by the strong westerly flow primarily determines the balloon dispersion. The mean square separation between balloons released 2 hours apart within the vortex (70¿S, 50 mbar) increases in time, following approximately (1) the Kraichnan-Lin exponential law for small timescales and space scales (1.5--4 days, 90--150 km), (2) the Richardson--Obukhov t3 law for intermediate timescales and space scales (7--18 days, 180--640 km), and (3) the asymptotic linear behavior found by , for large timescales and space scales (20--40 days, 700--1200 km). ¿ American Geophysical Union 1995