The development of genetic competence in Bacillus subtilis requires the synthesis of ComK, a transcription factor, which is normally produced as a culture enters the stationary phase. This synthesis is known to be regulated in part by the protein MecA. Loss-of-function mutations in mecA result in overexpression of ComK and its appearance early during exponential growth. We show here that mecA inactivation also causes a loss of colony-forming ability, especially during stationary phase. This loss is accompanied by the appearance of cells in which normal nucleoid separation has failed to occur. Renografin gradient fractionation of mecA cultures grown to competence reveals that nearly 100% of the cells band at the low buoyant density characteristic of competent cells, and that this low density is competence-related. The loss of viability, the low buoyant density and the nucleoid separation defect, are all comK-dependent. The loss of viability can be reversed by even the transient introduction of mecA+. It is proposed that these effects of ComK overexpression are related to the DNA replication arrest normally exhibited by the competent cell fraction and that MecA is needed to reverse this arrest and to permit escape from the competent state. The shift of nearly 100% of the cells to light buoyant density in a mecA mutant culture strongly suggests that the MecA protein is a regulator of the cell-type-specific expression of competence.