A survey was conducted in a freshwater reservoir located in Guizhou province, south China, to investigate the influence of fish aquaculture activities on mercury (Hg) distribution, speciation change, and bioaccumulation. Water, sediment, and aquaculture fish samples were collected from aquaculture sites (AS) and the corresponding reference sites (RS). The results showed that total mercury (THg) concentration in overlying water in Wujiangdu reservoir reached 6.87 +/- 14.9 ng L-1. THg concentration in a different layer follows the sequence: surface layer > bottom layer > three intermediate layers with significant difference. In addition, the total methylmercury (TMeHg) concentration in overlying water was 0.113 +/- 0.211 ng L-1. The highest TMeHg was observed in the bottom layer. Both dissolved Hg (DHg) and dissolved methylmercury (DMeHg) accounted for more than 50% of THg and TMeHg in the top four layers of overlying water, respectively. In contrast, particulate Hg (PHg) and particulate methylmercury were the major portion of THg and TMeHg in bottom layer and reached to 67 and 58.7% of THg and TMeHg, respectively. Aquaculture activities were estimated to contribute an annual loading of approximately 69.8 ng g(-1)of THg yearly in the top 1 cm of sediment underneath the rafts due to unconsumed fish feed. The extra loading of THg in sediment may have potential to stimulate the release of Hg to the overlying water. Both DMeHg and DHg in sediment pore water and organic matter (OM) contents in the top 4 cm of AS were noticeably higher than RS, which indicated that the accumulation of OM due to aquaculture activities promoted MeHg production in the top surface of sediment pore water. No significant difference was noted between THg in RS (296 +/- 104 ng g(-1)) and AS (274 +/- 132 ng g(-1)) in the solid phase. In addition, the sediments were net sources of both MeHg and inorganic Hg. For Hg in fish tissues, THg and inorganic Hg (IHg) in the liver were significantly higher (THg: p < 0.001, F = 9.98; IHg: p < 0.001, F = 13.1) than those in the other organs. In contrast, MeHg concentration in the muscle was significantly higher than (p < 0.01, F = 4.83) that in the other organs.