Bacillus subtilis is one of the most widely studied microorganisms. The organism can grow on minimal media, can use glycerol or acetate as a carbon source, and is a strict aerobe. Because of its ability to be transformed and because it can undergo transduction by bacteriophages, B. subtilis has a well-developed genetic map. In addition, the bacterium is frequently used to study cellular differentiation since certain growth conditions promote sporulation. The easily manipulated system for the exchange of DNA and the tendency of the organism to secrete copious quantities of proteins during growth also make B. subtilis an attractive candidate for commercial exploitation with the modem tools of genetic engineering. Figure 1 shows thin sections of B. subtilis undergoing cell division. A typical B. subtilis unit cell (the newly born or smallest cell in a population) has a girth of 1.8 km and a pole-to-pole length of 3 to 5 km.’.’Complete septa (Figure 1) are not rounded, but upon cell separation, the newly formed poles assume a shape intermediate between a flat surface and a hemisphere, making the cell ends somewhat blunt. During growth, B. subtilis appears to maintain a constant girth, regardless of generation time’(it should be pointed out, however, that cell width of B. subtilis has not been critically examined using electron microscopy of cells obtained at various growth rates). The length of the unit cell (newly born or smallest in the population), however, depends on growth rate. Bacilli dividing very rapidly (18 to 20 min is the most rapid doubling time of wildtype organisms at 37 to 45 C) are longer at birth than those doubling more slowly.’. 3 The maintenance of cellular circumference along the axis of the rod and throughout the division cycle is critical in understanding the strategy of the biophysics of growth of B. subtilis and probably for other Gram-positive bacilli as well. This article is largely concerned with how Gram-positive rods are able to elongate during growth processes and ultimately divide. Aspects of surface elongation in bacilli have been reviewed by K~ ch,~ Koch and Doyle,’Mendelson, 6 Munson and Glaser,’Reusch, 8 Sargent,’and Shockman and Barren.’In bacilli, there are several postulated modes of wall growth (Figure 2). Some have more support than others. Some depend totally on zonal growth, whereas others depend totally on diffuse insertion of new wall material. Most of the difficulty in clearly identifying how bacilli elongate comes from confusion in distinguishing between growth in cell cylinders and cell septa (nascent poles). In the conservative or apical case, one daughter cell inherits all of the surface of the mother cell, whereas the other daughter receives none. There is clear evidence for this mode in fungi in the literature, but none to support this means of surface growth for B. subtilis. lo In the semiconservative mode of surface extension, growth occurs by zones in which old wall is separated during growth processes and both daughters have one half of the old wall and one half new wall. This is an appealing and logical means for cell division in bacilli. Work in the laboratories of Higgins, Shockman, and Daneo-Moore has shown unequivocally that zonal growth occurs in streptococci, and, in fact, the growth zones provide markers to study cell cycle events. Streptococcus fuecium has been the most widely studied coccus in terms of surface growth, but the streptococci are not true cocci because their ends are slightly elongated and not round. Koch et aI. l6.’’have developed the biophysical basis of the zonal mode which seems to adequately describe the cell shapes of S. fuecium. This model is an application of the “surface stress theory” of …