Methylmercury (MeHg) produced by anaerobic bacteria in lakes and reservoirs poses a threat to ecosystem and human health due to its ability to bioaccumulate in aquatic food webs. This study used 48-hour microcosm incubations of profundal sediment and bottom water from a sulfate-rich, hypereutrophic reservoir to assess seasonal patterns of MeHg cycling under various treatments. Treatments included addition of air, inorganic mercury, organic carbon, and microbial inhibitors. Both aeration and sodium molybdate, a sulfate-reducing bacteria (SRB) inhibitor, generally decreased MeHg concentration in microcosm water, likely by inhibiting SRB activity. The methanogenic inhibitor bromoethanesulfonate increased MeHg concentration 2- to 4-fold, suggesting that methanogens were potent demethylators. Pyruvate increased MeHg concentration under moderately reduced conditions, likely by stimulating SRB, but decreased it under highly reduced conditions, likely by stimulating methanogens. Acetate increased MeHg concentration, likely due to the stimulation of acetogenic SRB. Results suggest that MeHg production at the sediment-water interface is elevated under moderately reduced conditions (-100 to -50 mV). In contrast, it is suppressed under oxic conditions due to low SRB activity, and under highly reduced conditions (> -100 mV) due to enhanced demethylation by methanogens.