The cell homogenate and the soluble cell fraction of Wolinella succinogenes grown with formate and fumarate catalyzed the oxidation of benzyl viologen radical by methacrylate [apparent Km=0.23 mM, Vmax=1.0 U (mg cell protein) -1] or acrylate [apparent Km=0.50 mM, Vmax=0.77 U (mg cell protein) -1]. Crotonate did not serve as an oxidant. A mutant of W. succinogenes lacking the fccABC operon was unable to catalyze methacrylate or acrylate reduction. In contrast, the inactivation of fccC alone had no effect on these activities. Methacrylate reduction by benzyl viologen radical was not catalyzed by fumarate reductase isolated from the membrane of W. succinogenes. Cells grown with formate and fumarate did not catalyze methacrylate reduction by formate, and W. succinogenes did not grow with formate and methacrylate as catabolic substrates. The results suggest that the reduction of methacrylate or acrylate by benzyl viologen radical is most likely catalyzed either by the periplasmic flavoprotein FccA or by a complex consisting of FccA and the predicted c-type cytochrome FccB. The metabolic function of the fccABC operon remains unknown.