Conventional treatment methods for arsenic removal from copper smelting wastewaters create sludge that is difficult to handle. Biosorption of arsenic using algae as sorbent is an interesting alternative to the conventional methods. This work shows results from biosorption of arsenic(V) by Lessonia nigrescens at pH=2.5, 4.5 and 6.5. The adsorption of arsenic could be explained satisfactorily both by the Freundlich and the Langmuir isotherms. Maximum adsorption capacities were estimated to 45.2 mg/g (pH=2.5), 33.3 mg/g (pH=4.5), and 28.2 mg/g (pH=6.5) indicating better adsorption at the lower pH. These values are high in comparison with other arsenic adsorbents reported. The sorption kinetics of arsenic by L. nigrescens could be modelled well by Lagergren's first-order rate equation. The kinetics were observed to be independent of pH during the first 120 min of adsorption with the Lagergren first-order rate constant of around 1.07x10(-3) min(-1). (C) 2005 Published by Elsevier Ltd.