The short-term accumulation of chromate by the cyanobacteriaAnabaena variabilis andSynechococcus PCC 6301 has been described as consisting of a rapid and relatively low level of biosorption of chromate to the cell walls; no energy-dependent uptake was detected. This biosorption was dependent on chromate concentration and could be described by a Freundlich adsorption isotherm for both cyanobacterial species studied. Decreasing the external pH increased the chromate accumulation by both species. Over a longer time period with growth it was shown thatA. variabilis was capable of reducing chromate (VI) to chromium (III) and then accumulating the chromium (III).Synechococcus PCC 6301 showed no further interaction with chromate concentrations over the same time period after the initial biosorption.