Recent studies have indicated that chemoautotrophic Epsilonproteobacteria might play an important role, especially as anaerobic or microaerophilic dark CO(2)-fixing organisms, in marine pelagic redoxclines. However, knowledge of their distribution and abundance as actively CO(2)-fixing microorganisms in pelagic redoxclines is still deficient. We determined the contribution of Epsilonproteobacteria to dark CO(2) fixation in the sulfidic areas of central Baltic Sea and Black Sea redoxclines by combining catalyzed reporter deposition-fluorescence in situ hybridization with microautoradiography using [(14)C]bicarbonate and compared it to the total prokaryotic chemoautotrophic activity. In absolute numbers, up to 3 x 10(5) (14)CO(2)-fixing prokaryotic cells ml(-1) were enumerated in the redoxcline of the central Baltic Sea and up to 9 x 10(4) (14)CO(2)-fixing cells ml(-1) were enumerated in the Black Sea redoxcline, corresponding to 29% and 12%, respectively, of total cell abundance. (14)CO(2)-incorporating cells belonged exclusively to the domain Bacteria. Among these, members of the Epsilonproteobacteria were approximately 70% of the cells in the central Baltic Sea and up to 100% in the Black Sea. For the Baltic Sea, the Sulfurimonas subgroup GD17, previously assumed to be involved in autotrophic denitrification, was the most dominant CO(2)-fixing group. In conclusion, Epsilonproteobacteria were found to be mainly responsible for chemoautotrophic activity in the dark CO(2) fixation maxima of the Black Sea and central Baltic Sea redoxclines. These Epsilonproteobacteria might be relevant in similar habitats of the world's oceans, where high dark CO(2) fixation rates have been measured.