A crucial step in converting an actively growing Bacillus subtilis cell into a dormant spore is the formation of a cell within a cell. This unusual structure is created by a phagocytosis-like process in which the larger mother cell progressively engulfs the adjacent smaller forespore. Only mutations blocking engulfment at an early stage and affecting genes expressed in the mother cell have been identified. Here we describe a new locus, spoIIQ, which is transcribed in the forespore and which encodes a membrane-bound protein required at a late stage of engulfment. Immunofluorescence microscopy analysis have shown that SpoIIQ is initially targeted to the septum at the boundary between the two cells and then spreads around the entire membrane of the forespore. Septum targeting requires only the first 52 residues of SpoIIQ as well as unidentified forespore-specific components. Electron-microscopy studies of cells engineered to activate the mother-cell program of gene expression independently of the forespore indicate that other as yet uncharacterized genes are involved in engulfment and that this morphological process is driven from both sides of the forespore envelope.