A simple approachto controlthe vonKarman vortexstreet behind a two-dimensionalcircular cylinderbased on the proportional feedback of the estimate of just the Ž rst POD mode has been developed. A stabilityanalysisof this control lawwas conductedafter linearization about the desired equilibriumpoint (origin) and conditions for controllabilityand asymptotic stabilitywere developed.The control approach, applied to the four-mode cylinder wake POD model at a Reynolds number of 100 stabilizes the wake for a proportionalgain above1.6.Whereas the controlleruses only the estimatedamplitude of the Ž rst mode, all four modes are stabilized. This suggests that the higher-ordermodes are caused by a secondary instability.Thus, they are suppressed once the primary instability is controlled. The control approachwill be further examined to observe its sensitivity to time delays, actuator limitations,modeling and estimationerrors, and sensor noise.