Pyrimidine base and ribonucleoside utilization was investigated in the two type strains of the Pseudomonas alcaligenes group. As sole sources of nitrogen, the pyrimidine bases uracil, thymine and cytosine as well as the dihydropyrimidine bases dihydrouracil and dihydrothymine supported the growth of Pseudomonas pseudoalcaligenes ATCC 17440 but neither these bases nor pyrimidine nucleosides supported Pseudomonas alcaligens ATCC 14909 growth. Ribose, deoxyribose, pyrimidine and dihydropyrimidine bases as well as pyrimidine nucleosides failed to be utilized by either P. pseudoalcaligenes or P. alcaligenes as sole carbon sources. The activities of the pyrimidine salvage enzymes nucleoside hydrolase, cytosine deaminase, dihydropyrimidine dehydrogenase and dihydropyrimidinase were detected in cell-free extracts of P. pseudoalcaligenes and P. alcaligenes. In P. pseudoalcaligenes, the levels of cytosine deaminase, dihydropyrimidine dehydrogenase and dihydropyrimidinase could be affected by the nitrogen source present in the culture medium.