Significance Many bacteria form a thick cell wall and divide by forming a cross wall. How they control the thickness of their cell wall and cross wall is unknown. In this study, we show that in these bacteria the cell division protein SepF forms very large protein rings with diameters that correspond to the diameter of their cross walls. Importantly, when we changed the diameter of SepF rings in the bacterial host Bacillus subtilis , the thickness of its cross wall changed accordingly. These results provide strong evidence that a large protein ring can function as a mold to control the thickness of the cell wall that divides these bacterial cells.