Accumulation and toxicity of inorganic arsenic forms As(III) and As(V) to the green microalga Stichococcus bacillaris depended on environmental variables. pH of exposure and to a lesser extent elevated concentrations of humic acid, chloride and orthophosphate ions affected arsenic accumulation and its toxicity. As(V) was more toxic than As(III), especially at the near neutral pH 6.8. Intracellular As(V) uptake by algal cells was greater at pH 6.8 than at 8.2. In response to As(III) and As(V) the alga produced phytochelatins (PC(2-3)), but at As(V) exposure, their levels in cells were higher than with As(III), suggesting higher As(V) than As(III) availability and uptake. Arsenic in algal cells occurred in various complexes with non-protein SH groups. Some of these complexes dissociated under acidic conditions, but others were able to dissociate only at an alkaline pH. The former consisted of SH groups of phytochelatins. Those dissociating at an alkaline pH involved SH groups from both glutathione (GSH) and phytochelatins (PC) or their derivatives. In the predominant acid-stable mixed As-SH complex, the ratio of SH (PC(2)) to SH (GSH) was 2:1, which suggests that one molecule of PC(2) (containing two SH groups) together with one molecule of GSH were involved in intracellular complexation of each As atom. This is the first demonstration of GSH involvement in arsenic complexation, in vivo. The intracellular concentration of As was greater than that of non-protein SH groups which suggests that not all the arsenic in algal cells was complexed and detoxified by thiol groups.