Selenium is an essential component of several enzymes and proteins in a number of methane-producing archae. Information concerning accessory proteins that function in selenium transport processes, however, is limited. A novel selenium-binding protein with a potential transport role and a selenocysteine lyase that serves as a selenium delivery protein are present in Methanococcus vannielii. The selenium-binding protein was purified from extracts of 75Se-labeled cells. Although there was gradual loss of 75Se during purification, the isolated protein still could be detected as a radioactive 42 kDa species on native PAGE gels and as a 33 kDa species on SDS PAGE gels. The N-terminal amino acid sequence of residues 1–63 of the protein was determined by automated Edman degradative analysis. The only homologous sequence detected in the recorded data base was that of a gene encoding an unknown protein located in the genomic sequence of Methanococcus maripaludis. Cloning and expression of the corresponding gene from M. vannielii are described in a manuscript in press (Self et al.). A 47 kDa selenocysteine lyase isolated from M. vannielii extracts exhibited sequence homology to the NIFS family of proteins that transport sulfur. The purified selenocysteine lyase catalyzed the elimination of an elemental form of selenium from free selenocysteine and delivered this selenium directly to selenophosphate synthetase. The synthetase converted the selenium to selenophosphate in an ATP-dependent reaction.