Cobalt-60 is a by-product formed in the nuclear and defense-related industries. Bioreduction of stable [Co(III)-EDTA](-) to the less stable [Co(II)-EDTA](2-) constitutes a bioremediation approach. Bioreduction of [Co(III)-EDTA](-) by a denitrifying microbial community was studied. Denitrifying granular sludge cultivated in a bioreactor exhibited efficient [Co(III)-EDTA](-) reduction. Fed-batch experiments showed repeated reduction of [Co(III)-EDTA](-) without loss of microbial activity. Bioreduction of [Co(III)-EDTA](-) was accelerated in the presence of anthraquinone-2,6-disulfonate. Simultaneous reduction of both nitrate and [Co(III)-EDTA](-) was observed, but denitrification was delayed in the presence of [Co(III)-EDTA](-). Microbial reduction in the presence of excess nitrate and acceleration by an electron shuttle has practical implications in cobalt bioremediation.