Total mercury (THg), methylmercury (MeHg) and environmental factors were determined to study distributions of MeHg and Hg methylation in the sediments from 8 main mangrove areas of China. The results showed that it was not consistent for distributions of THg and MeHg in sediments. Concentrations of MeHg in sediments from Sanya, Dongzhaigang (Hainan Province), Techengdao, Leizhou, Gaoqiao, Futian (Guangdong Province), Daguansha (Guangxi Autonomous Region), Fugong (Fujian Province) were (0.24 +/- 0.04), (0.58 +/- 0.27), (0.52 +/- 0.23), (1.56 +/- 0.49), (0.50 +/- 0.25), (1.21 +/- 0.36), (1.86 +/- 1.04), (0.47 +/- 0.16) ng x g(-1) respectively. There were regional difference in MeHg contents which decreased in the order of Daguansha > Leizhou > Futian > Dongzhaigang > Techengdao > Gaoqiao > Fugong > Sanya. Input of Hg and organic matter from industry and aquiculture may lead to high level of MeHg. Compared with sediments from other estuaries of the world, serious pollution of MeHg was found in mangrove sediments of China. (2) % MeHg in mangrove sediments ranged from 0.11% to 7.13%, which decreased in the order of Daguansha > Techengdao > Leizhou > Futian > Gaoqiao > Dongzhaigang > Sanya > Fugong. There was significantly positive correlation between % MeHg and sandy fraction (p < 0.05), and significantly negative correlation between % MeHg and silt-clay fraction (p < 0.05). (3) Total bacteria (TB) in mangrove sediments ranged from 2.44 x 10(10) to 1.91 x 10(11) CFU/g (dry weight), TB decreased in the order of Fugong > Sanya > Gaoqiao > Dongzhaigang > Futian > Daguansha. Sulfate-reducing bacterium (SRB) ranged from 1.73 x 10(4) to 4.92 x 10(6) CFU/g, SRB decreased in the order of Futian > Fugong > Dongzhaigang > Sanya > Gaoqiao > Daguansha. Wastewater with high organic matters leads to high SRB. The types of surface sediments also had a great impact on the amount of SRB. There was no significant correlation among MeHg and environmental factors, which indicated that exogenous input is the main cause of MeHg pollution in mangrove sediments.