Methanococcus vannielii is capable of degrading purines to the extent that each of these purines may serve as the sole nitrogen source for growth. Results presented here demonstrate that purine degradation by M. vannielii is accomplished by a route similar to that described for clostridia. Various characteristics of the purine-degrading pathway of M. vannielii are described. Additionally, it is shown that M. vannielii does not extensively degrade exogenously supplied guanine if that compound is present at levels near or lower than those required to supply the cellular guanine requirement. Under those conditions, M. vannielii incorporates the intact guanine molecule into its guanine nucleotide pool. The benefits of a purine-degrading pathway to methanogens are discussed.