The biological requirement of the trace element selenium was recognized 40 years ago. Selenium is incorporated into several enzymes and transfer RNA species of both prokaryotic and eukaryotic origin. In enzymes which contain a selenopolypeptide, selenium is present as covalently bound selenocysteine which participates in the catalytic reaction. Sequence analysis of the genes coding for two selenoproteins, formate dehydrogenase H from Escherichia coli and glutathione peroxidase from mouse and man, demonstrated that an in-frame UGA opal nonsense codon directs the incorporation of selenocysteine. In the case of formate dehydrogenase incorporation occurs cotranslationally. Recently, we identified four genes whose products are required for selenocysteine incorporation in E. coli. We report here that one of these genes codes for a tRNA species with unique properties. It possesses an anticodon complementary to UGA and deviates in several positions from sequences, until now, considered invariant in all tRNA species. This tRNA is aminoacylated with L-serine by the seryl-tRNA ligase which also charges cognate tRNASer. Selenocysteine, therefore, is synthesized from a serine residue bound to a natural suppressor tRNA which recognizes UGA.