The effects of various exogenous nucleic acid compounds on the viability and cell composition of Bdellovibrio bacteriovorus starved in buffer were measured. In decreasing order of effectiveness, these compounds were found to decrease the rate of loss of viability and the loss of cell carbon, cell ribonucleic acid, and cell protein: glutamate > ribonucleoside monophosphates > ribonucleosides > deoxyribonucleoside monophosphates. Similar sparing effects were not observed with nucleic acid bases, deoxyribonucleosides, ribose, ribose-5-phosphate, deoxyribose, and deoxyribose-5-phosphate. Appreciable increases in the respiration rate over the endogenous rate did not occur when cell suspensions were incubated with individual or mixtures of nucleic acid compounds. Formation of 14CO2 by cell suspensions incubated with carbon 14-labeled nucleic acid compounds indicated ribonucleosides and ribonucleoside monophosphates were respired and to a small extent, were incorporated into cell material of non-growing cells. The respired 14CO2 was derived mainly from the ribose portion of these molecules. No respired 14CO2 or incorporated carbon 14 was found with bdellovibrios incubated with other nucleic acid compounds tested, including free ribose. During growth of B. bacteriovorus on Escherichia coli in the presence of exogenous UL-14C-ribonucleoside monophosphates, 1 0 -16 ~ of the radioactivity was in the respired CO2 and of the radioactivity incorporated into the bdellovibrios, only 40 to 50 ~ resided in the cell nucleic acids. However, during growth on 14C-adenine, -uracil, or -thymidine labeled E. coli, Abbreviations. HM buffer = 5 mM N-2-hydroxyethyl-piperazine-N'-2-ethanesulfonic acid (pH 7.6) containing 0.1 mM CaClz and MgC12; DNA = deoxyribonucleic acid; RNA = ribonucleic acid; Ar, Cr, Gr, Ur = ribonucleosides of adenine, cytosine, guanine, uracil, respectively; dTr = deoxythymidine; AMP, CMP, GMP, UMP = ribonucleoside monophosphates of adenine, cytosine, guanine, and uracil, respectively; dTMP = deoxythymidine monophosphate; ATP = adenosine triphosphate; PFU = plaque-forming units only trace amounts of 14CO2 were found and 90 ~o or more of the incorporated radioactivity was "in the bdellovibrio nucleic acids. It is concluded that bdellovibrio can use ribonucleoside monophosphates during growth and starvation as biosynthetic precursors for synthesis of both nucleic acids and other cell materials as well as catabolizing the ribose portion for energy purposes.