In this study the degradation of extracellular purines by the bacterium Paracoccus denitrificans was examined with aqueous purine solutions. Paracoccus denitrificans was able to decompose free purine bases and 5'-mononucleotides. The nitrogen-containing products of the degradation were ammonia and urea. Purine uptake was the main control of purine decomposition. In the cases of guanine, xanthine, hypoxanthine, and urate, further control was exerted by induction. Furthermore, the uptake of the purines caused differences in the duration and temporal development of the substrate degradation. It was also responsible for the inhibitory effects of the purines on the decomposition of one another when the substrates were used in mixtures. Also, fermentation parameters like biomass and purine concentration, pH, and temperature influenced the purine usage of Paracoccus denitrificans.