The obligate intracellular parasite, Rickettsia prowazekii, is a slow-growing bacterium with a doubling time of about 10 h. In the present study, DNA and RNA were obtained from the rickettsiae by two independent methods, i.e. simultaneous isolation of DNA and RNA from the same sample by phenol:chloroform extraction and CsCI gradient centrifugation. In addition, ribosomal RNA was obtained by sedimentation of partially purified ribosomes from the rickettsiae. The results demonstrated that, after correction for the cell volumes, the concentrations of stable RNA and ribosomes in R. prowazekii, a slow-growing organism, were about 62 fg micron-3 and 17,000 per micron3, respectively, which were very similar (66 fg micron-3 and 21,000 per micron3) to those in Escherichia coli with a generation time of 40 min. However, on a per cell basis, R. prowazekii had 5.6 fg of RNA and 1500 ribosomes per cell, which was only about 8% of the amount of both stable RNA (71.2 fg) and ribosomes (24,000) per cell as was found in E. coli. These results indicated that R. prowazekii possesses a ribosome concentration greater than might have been predicted from its slow growth rate. This high concentration of ribosomes could be due to a large population of nonfunctioning ribosomes, a low efficiency of amino acid production, or a high rate of protein turnover. However, this study also demonstrated that the rickettsiae have very limited protein turnover. Knowledge of the kinetics and control mechanisms for protein synthesis in R. prowazekii remains to be established to determine the logic of the extra rickettsial ribosomes.