Arsenic (As) is a pervasive and ubiquitous environmental toxin that has created world-wide human health problems. However, there are few studies about how organisms detoxify arsenic. Cyanobacteria are capable of both photolithotrophic  growth in the light and heterotrophic growth in the dark and are ubiquitous in soils,aquatic systems and wetlands. In this study, we investigated arsenic  biotransformation in three cyanobacterial species (Microcystis sp. PCC7806, Nostoc sp. PCC7120 and Synechocystis sp. PCC6803). Each accumulated large amounts of arsenic, up to 0.39 g kg-1 dry weight (DW), 0.45 g kg-1 DW, and 0.38 gkg-1 DW when treated with 100 μM sodium arsenite for 14 days, respectively.Inorganic arsenate and arsenite were the predominant species, with arsenate making up more than 80% of total arsenic; methylated arsenicals were detected following exposure to higher arsenic concentrations. When treated with arsenate for six weeks, cells of each cyanobacteria produced volatile arsenicals. The genes encoding the ArsM (As(III) S-adenosylmethionine methyltransferase) were cloned from these three cyanobacteria. When expressed in an arsenic-hypersensitive strain of Escherichia coli, each conferred resistance to arsenite. Two of ArsM homologues (SsArsM from Synechocystis sp. PCC6803 and NsArsM from Nostoc sp. PCC7120) were purified, and were shown to methylate arsenite in vitro with trimethylarsine as the end product. Given that ArsM homologues are widespread in cyanobacteria, we propose that they 49 play an important role in arsenic biogeochemistry.