We investigated the production of manganese (Mn) oxides using repeated-batch bioreactors maintained over long periods under laboratory conditions. Freshwater epilithic biofilms were used as the initial inocula. The bioreactors yielded suspended solids that could remove 0.1 mM dissolved Mn(II) within a few days. Chemical titration, X-ray absorption near-edge structure spectroscopy, and X-ray diffraction analysis revealed that the Mn(II) had been converted to poorly crystallized layer-type Mn(IV) oxides, which were similar to known, biogenic Mn oxides from pure bacterial cultures. Spherical or rod-shaped Mn microconcretions occurred in the suspended solids; transmission electron microscopy showed that these structures likely resulted from the microbial activity but not represent living cells. Instead, the presence of encapsulated, sheathed, and hyphal budding cells in the suspended solids indicated that a range of Mn-depositing bacteria contributed to the Mn oxide formation. To our knowledge, our data represent the first observation of production of such Mn oxides in a laboratory microcosm wherein a range of Mn-depositing bacteria coexist. The fact that sorption of trace Zn(II) and Ni(II) ions onto the suspended solids co-occurred with the removal of dissolved Mn(II) emphasizes the important role of Mn-oxidizing microorganisms in the fates of trace or contaminant metals in the aquatic environment.