In the phylogenetic tree, selenoproteins and the corresponding translation machinery are found in Archaea, Elebacteria, and animals, but not in fungi and higher plants. As very little is known about Protozoa, we searched for the presence of selenoproteins in the primitive dinoflagellate Oxyrrhis marina, belonging to the Protoctista kingdom. Four selenoproteins could be obtained from O. marina cells cultured in the presence of Se-75. Using O. marina or bovine liver cytosolic extracts, we Could serylate and selenylate in vitro total O. marina tRNAs. Moreover, the existence of a tRNA(Sec) could be deduced from in vivo experiments, Lastly, an anti-serum against the specialized mammalian translation elongation factor mSelB reacted with a protein of 48-kDa molecular mass. Altogether, our data showed that O. marina contains selenoproteins and suggests that the corresponding translation machinery is related to that found in animals, (C) 2002 Elsevier Science (USA). All rights reserved.