Pyrrolysine, the twenty-second amino acid found to be encoded in the natural genetic code(1-4), is necessary for all of the known pathways by which methane is formed from methylamines(5,6). Pyrrolysine comprises a methylated pyrroline carboxylate in amide linkage to the e-amino group of L-lysine(2,7,8). In certain Archaea, three methyltransferases initiate methanogenesis from the various methylamines(9-11), and these enzymes are encoded by genes with an in-frame amber codon(12,13) that is translated as pyrrolysine(2,7,8). Escherichia coli that has been transformed with the pylTSBCD genes from methanogenic Archaea can incorporate endogenously biosynthesized pyrrolysine into proteins(14). The decoding of UAG as pyrrolysine requires pylT(1,6), which produces tRNA(Pyl) (also called tRNA(CUA)), and pylS(1), which encodes a pyrrolysyl-tRNA synthetase(4,15,16). The pylB, pylC and pylD genes(1) are each required for tRNA-independent pyrrolysine synthesis(14). Pyrrolysine is the last remaining genetically encoded amino acid with an unknown biosynthetic pathway. Here we provide genetic and mass spectrometric evidence for a pylBCD-dependent pathway in which pyrrolysine arises from two lysines. We show that a newly uncovered UAG-encoded amino acid, desmethylpyrrolysine, is made from lysine and exogenous D-ornithine in a pylC-dependent process followed by a pylD-dependent process, but it is not further converted to pyrrolysine. These results indicate that the radical S-adenosyl-L-methionine (SAM) protein PylB mediates a lysine mutase reaction that produces 3-methylornithine, which is then ligated to a second molecule of lysine by PylC before oxidation by PylD results in pyrrolysine. The discovery of lysine as the sole precursor to pyrrolysine will further inform discussions of the evolution of the genetic code and amino acid biosynthetic pathways. Furthermore, intermediates of the pathway may provide new avenues by which the pyl system can be exploited to produce recombinant proteins with useful modified residues.