The stratiform sedimentary manganese ore deposit at Urkut, Hungary contains primary ore that is exclusively manganese carbonate. It shares many geological, sedimentological, mineralogical, and geochemical characteristics with other organic-rich, sediment-hosted, marine manganese deposits. Samples from two stratigraphic sequences through the marlstone-hosted Mn carbonate ore at Urkut were studied. Stable isotope compositions of both Mn-rich and Mn-poor carbonates were analyzed and are discussed, together with the geologic background of the deposit, rock and mineral chemistry, and x-ray diffraction mineralogy. The delta 13 C values of carbonates (-1.24 to -30.78 per thousand ) show a negative linear correlation with Mn contents and a negative exponential correlation with total organic carbon contents. These relations suggest that the Urkut deposit was produced by early diagenetic precipitation of manganese carbonate. Data are consistent with models for bacterial metabolic pathways. Thus, mineralization was a consequence of bacterially mediated diagenetic reactions that involved Mn reduction via one or two coupled reactions: the oxidation of organic matter with Mn oxyhydroxide reduction, or the oxidation of FeS produced as a byproduct of seawater sulfate reduction with Mn oxyhydroxide reduction. Oxygen-deficient water in the depositional basin acted as a carrier and reservoir for Mn (super +2) before its deposition.