SELENOCYSTEINE is incorporated cotranslationally at UGA codons, normally read as stop codons, in several bacterial proteins^1,2 and in the mammalian proteins glutathione peroxidase (GPX)^3–5, selenoprotein P⁶ and Type Iiodothyronine 5′ deiodinase (5′DI)⁷. Previous analyses in bacteria have suggested that a stem–loop structure involving the UGA codon and adjacent sequences is necessary and sufficient for selenocysteine incorporation into formate dehydrogenase and glycine reductase^2,8,9. We used the recently cloned 5′DI to investigate selenoprotein synthesis in eukaryotes. We show that successful incorporation of seleno-cysteine into this enzyme requires a specific 3′ untranslated (3′ut) segment of about 200 nucleotides, which is found in both rat and human 5′DI messenger RNAs. These sequences are not required for expression of a cysteine-mutant deiodinase. Although there is little primary sequence similarity between the 3′ut regions of these mRNAs and those encoding GPX, the 3′ut sequences of rat GPX can substitute for the 5′DI sequences in directing selenocysteine insertion. Computer analyses predict similar stem-loop structures in the 3′ut regions of the 5′DI and GPX mRNAs. Limited mutations in these structures reduce or eliminate their capacity to permit 5′DI translation. These results identify a 'selenocysteine-insertion sequence' motif in the 3′ut region of these mRNAs that is essential for successful translation of 5′DI, presumably GPX, and possibly other eukaryotic selenocysteine-containing proteins.