Placement of the Z ring at midcell in Escherichia coli is assured by the action of the min system, which blocks usage of potential division sites that exist at the cell poles. This activity of min is achieved through the action of an inhibitor of division, MinC, that is activated by MinD and topologically regulated by MinE. In this study, we have used a functional GFP–MinC fusion to monitor the location of MinC. We find that GFP–MinC is a cytoplasmic protein in the absence of the other Min proteins. The addition of MinD, a peripheral membrane protein that interacts with MinC, results in GFP–MinC appearing on the membrane. In the presence of both MinD and MinE, GFP–MinC oscillates rapidly between the halves of the cell. Thus, MinC is positioned by the other Min products, but in a dynamic manner so that it is in position to inhibit Z ring assembly away from midcell.