METHYLMERCURY can accumulate in fish to concentrations that threaten human health(1). Fish methylmercury concentrations are high in many reservoirs(2) and acidic lakes(3), and also in many remote lakes(4,5)-a fact that may be related to increased atmospheric deposition of anthropogenically mobilized mercury during the past few decades(6). Although sources of methylmercury to lakes and reservoirs are known(7), in-lake destruction has not been demonstrated to occur at the low concentrations found in most water bodies. Here we report in situ incubations of lake water that show that methylmercury is decomposed by photo-degradation in surface waters. This process is abiotic and the rate is first-order with respect to methylmercury concentration and the intensity of solar radiation. In our study lake, the calculated annual rates of methylmercury photodegradation are almost double the estimated external inputs of methylmercury from rain, snow, streamflow and land runoff, implying the existence of a large source of methylmercury from bottom sediments. Photodegradation could also be an important process in the mercury cycle of other aquatic systems. This discovery fundamentally changes our understanding of aquatic mercury cycling, and challenges the long-accepted view that microbial demethylation dominates methylmercury degradation in natural fresh waters.