Most bacterial RNA consists of stable RNA which is composed of rRNA and tRNA. We have followed by primer extension analysis the level of ribosomal RNA synthesis along the growth phases of a cell culture. A sharp drop in rRNA synthesis was observed upon the transition from the exponential to the stationary phase of growth. Our results demonstrate that an effective shut-off of rRNA synthesis occurs also in the absence of ppGpp. Mutations in the host factors Fis and H-NS, which are known to regulate rrn P1 promoters, did not affect the shut-off process of ribosomal RNA promoters. We also tested the effect of RpoS, the sigma factor known to induce a number of genes in the stationary phase. It was shown that the host factors Fis, H-NS and RpoS do not play a major role in the regulation of the shut-off process of rRNA synthesis. The results presented demonstrate that the rate of rRNA synthesis provides a sensitive measure of the growth phase of the bacterial culture.