Sporulation in Bacillus subtilis is a simple form of differentiation that involves the formation of a two-cell organism; mother cell and prospective spore. It is regulated by at least 50 operons, some mono- and some polycistronic. These are expressed in a dependent sequence which branches at an early stage so that there are independent lines of expression in both cells; further ramifications then occur in both cell compartments. Three DNA-binding proteins and two sigma factors have so far been identified as probable regulators of the dependent sequence. Most of the operons of the sequence are expressed in the first four hours and the last two stages occur mainly by self-assembly of proteins.