tArsenic (As) is extremely toxic to living organisms at high concentration. In aquatic systems, As exists indifferent chemical forms. The two major inorganic As (iAs) species are AsV, which is thermodynamicallystable in oxic waters, and AsIII, which is predominant in anoxic conditions. Photosynthetic microorgan-isms (e.g., phytoplankton and cyanobacteria) take up AsV, biotransform it to AsIII, then biomethylate itto methylarsenic (MetAs) forms. Although AsIIIis more toxic than AsV, AsIIIis much more easily excretedfrom the cells than AsV. Therefore, majority of researchers consider the reduction of AsVto AsIIIas adetoxification process. The biomethylation process results in the conversion of toxic iAs to the lesstoxic pentavalent MetAs forms (monomethylarsonate; MMAV, dimethylarsonate; DMAV, and trimethy-larsenic oxide; TMAOV) and trimethylarsine (TMAOIII). However, biomethylation by microorganisms alsoproduces monomethylarsenite (MMAIII) and dimethylarsenite (DMAIII), which are more toxic than iAs, asa result of biomethylation by the microorganisms, demonstrates the need to reconsider to what extentAs biomethylation contributes to a detoxification process. In this review, we focused on the discussionof whether the biotransformation of As species in microorganisms is really a detoxification process withrecent data.