The selenium-containing F420-reducing hydrogenase from Methanococcus voltae was anaerobically purified to a specific hydrogen-uptake activity of 350 U/mg protein as determined with the natural electron acceptor. The concentrated enzyme was used for EPR-spectroscopic investigations. As isolated, the enzyme showed an EPR spectrum with gnyL values of 2.21, 2.15 and 2.01. Illumination of such samples at low temperatures led to an EPR spectrum with gxyz values of 2.05, 2.11 and 2.29. These spectra are typical for [NiFe]hydrogenases in the active state. Spectra of samples enriched in 77Se showed a hyperfine interaction between the unpaired spin of the nickel ion and the nuclear spin of one 77Se atom before and after illumination. A 900 flip of the electronic z-axis is proposed to explain the hyperfine interaction in both states. This has been demonstrated previously only for the F420-non-reducing hydrogenase from M. voltue, where the selenium atom is present as a selenocysteine residue on an unusually small seperate subunit [Sorgenfrei, O., Klein, A. & Albracht, S. P. J. (1993) FEBS Lett. 332, 291–2971. The results demonstrate that the three-dimensional structures of the active sites in the selenium-containing F420-reducing and F420-non-reducing hydrogenases from M. voltue are highly similar and hence are not influenced by the unusual subunit structure of the latter enzyme. Oxidized samples containing either natural selenium or 77Se were prepared from the F420-reducing and the selenium-containing F420-non-reducing hydrogenase. Both enzymes exhibited EPR spectra typical for [NiFe]hydrogenases in the inactive ‘ready’ state. In contrast to the reduced form, no splitting of the nickel-derived signal due to the nuclear spin of 77Se was observed in the oxidized state, indicating that the electronic z-axis is perpendicular to the Ni-Se direction.