Cells of Bacillus subtilis RodB changed from rods to cocci when shifted from 20 to 42 degrees C in media containing no additional anions. Quantitative studies of surface growth, including cross-wall formation and pole construction, have been made from reconstructions obtained from central, longitudinal sections of cells. Measurements of surface area and volume were obtained by mathematical rotation of axial sections about their longitudinal axis. Surface markers, perhaps analogous to the wall bands of streptococci, have been used to distinguish septal from cylindrical wall. During the shape change, wall volume increased most rapidly, in relation to cell volume, at the division site. The average volume of wall distal to the septum also increased but the slopes of the lines relating distal wall volume to cell volume were the same at all stages of the shape change. The quantity of wall per distal pole gradually declined with increase of cell volume and as the cells became more coccus-like. Collectively, these features suggest that wall continues to be produced at sites of cylindrical extension but fails to become incorporated into the existing cylinder to maintain a constant diameter. Instead, wall material may be used to thicken the surface of distal poles, but the rate of addition may decline as the cells assume a coccal morphology. The change from rods to cocci may involve a progressive dependence on septal growth for surface expansion and a modification of the time at which the cross-wall is closed. Septal closure is progressively delayed as the organisms change into cocci, so that cross-wall separation precedes septal closure, as in streptococci.