Metabolic differences in the strategies used for pyrimidine base and nucleoside salvage were studied in the pseudomonads and enteric bacteria. Fluoro--analogs were used to select mutant strains of E. coli, S. typhimurium, P. putida, and P. aeruginosa blocked in one or more of the uracil and uridine salvage enzymes. HPLC analysis of cell-free extracts from wild-type and mutant strains examined the effectiveness of the selections. Evidence was found for cytidine kinase in Pseudomonas and for an activity that converted uracil compounds to cytosine compounds. Using media supplemented with 150 μg of orotic acid per ml, P. putida SOC 1, a Pyr, upp mutant which utilizes orotic acid as a pyrimidine source was isolated for the first time in any study.