The evolution of the dynamical signal of an isolated Mediterranean Water lens ('meddy') is simulated using a quasigeostrophic model on the &bgr;-plane. The initially circular anticyclonic vortex moves south-westward at a non-uniform velocity of about 1 cm/s for over two years. Repeated instability processes (both barotropic and baroclinic) modify the horizontal shape and the radius of the meddy as well as its vertical structure. Particle trajectories inside the lens show strong structural similarities with tracks of 'SOFAR' floats deployed in a meddy by Armi et al. (1988). A tracer concentration inside the meddy is transported away from the source region and slowly spreads to the surrounding. 'SOFAR' floats injected in nearly zonal tracer tongues produced by the quasi-barotropic far field behind the meddy moved westward in a narrow band as observed by Prince et al. (1986). ¿ American Geophysical Union 1989