Thermophilic cyanobacterial strains that could grow in the BG I I media was isolated from hot springs and their reactive dye bioaccumulation was studied under thermophilic conditions in a batch system, in order to determine the optimal conditions required for the highest dye accumulation. In the experiments performed with newly isolated Synechococcus sp. and Phormidium sp., the optimum pH value of each microorganisms at about 25 mg 1(-1) initial reactive dye concentration was determined as 8.5 for Remazol Blue and Reactive Black B dyes and 9.5 for Reactive Red RB dye. At the optimum pH value of each dye, dye bioaccumulation by thermophilic cyanobacteria was investigated between 10 mg 1(-1) and 78 mg 1(-1) initial dye concentrations at 45 degrees C. At the end of the experiments, Phormidium sp. was shown high dye bioaccumulation than the Synechococcus sp., with maximum uptake yields ranging from 13% to 97% at all tested dye concentrations. In the experiments with Synechococcus sp., the maximum uptake yields were ranged from 8% to 66% for all dye concentrations. When the effect of temperatures on dye bioaccumulation were investigated, the highest uptake yields at high dye concentrations for Synechococcus sp. was measured as 23.0% for 78.3 mg 1(-1) Reactive Red RB, 39.9% for 72.4 mg 1(-1) Remazol Blue and 13.7% for 62.0 mg 1(-1) Reactive Black B at 40 degrees C temperatures. At the same temperature, the uptake yield of Phormidium sp. varied between 38.5% and 40.7% at 75 mg 1(-1) initial dye concentration. When the high temperature (50 degrees C) was used, Phormidium sp. was showed to have high dry weight and bioaccumulation yield than the Synechococcus sp., which can be considered as satisfactory in terms of dye removal from subject wastewater effluents under thermophilic conditions. (c) 2005 Elsevier Ltd. All rights reserved.