A cyanobacterium, Oscillatoria sp., isolated from the Mazandaran Rivers, Iran, was studied for its ability to eliminate Co(II) ions from aqueous solutions. Optimum conditions for biosorption of Co(II) ions by Oscillatoria sp. were investigated in terms of critical parameters such as pH, temperature, contact time, biomass concentration, initial metal concentration and influence of co-ions. Dried biomass of Oscillatoria sp. exhibited higher biosorption capacity than wet biomass. The maximal Co(II) ion biosorption capacities of the dried and wet biomass were recorded at pH 7 and 5, respectively. The range of initial Co(II) ion concentration tested was 5—200 mg/L and the experimental biosorption data was found to fit the Langmuir model better than the Freundlich model. Maximum biosorption capacity calculated for dried biomass of Oscillatoria sp. was 30.12 ± 0.10 mg/g based on the Langmuir model and optimum conditions were pH 7, 25°C, 0.08 mg/mL of biomass, 50 mg/L initial Co(II) and 6 h of contact time. Biosorption of Co(II) was reduced in the presence of equimolar amounts of co-ions. Lastly, the capacity of the dried biomass was tested for removal of Co(II) from river water samples supplemented with ∼14 mg/L Co(II), under the optimized experimental conditions.