Methanococcus maripaludis possesses two sets of F₄₂₀-non- reducing hydrogenases which are differentially expressed in response to the selenium content of the medium. One of the subunits of the selenium-containing hydrogenase, VhuD, contains two selenocysteine residues, whereas the homologue of M. voltae possesses cysteine residues in the equivalent positions. Analysis of the 3′ non-translated region of the M. voltae vhuD mRNA revealed the existence of a structure resembling the consensus of archaeal SECIS elements but with deviations rendering it non-functional in determining selenocysteine insertion. The presence of a pseudo-SECIS element in the 3′ non-translated region of the vhuD mRNA from M. voltae suggests that VhuD from this organism has developed from a selenocysteine-containing ancestor. The 3′ non-translated region from the VhcD homologues neither contained a SECIS nor a pseudo SECIS element.