The use of Saccharomyces cerevisiae was evaluated as substrate for the biosorption of As(III) in the presence of As(V), for the determination of these species in aqueous solutions. Experiments were performed in batch to find a suitable and simple method to obtain information on arsenic uptake. The effect of chemical and physical variables affecting the biosorption of As were tested in order to select the optimal analytical conditions for the selective As retention by Saccharomyces cerevisiae. The parameters studied were: reaction media and pH, amount of biomass, analyte concentration and contact time. The influence of some concomitant ions up to a concentration of 50 mg l^-1 was also tested. Quantitative determinations of As species in yeast and supernatant solution were carried out by means of inductively coupled plasma atomic emission spectrometry with hydride generation. During all the steps of the optimization process, As(V) remained in solution while As(III) was accumulated by the yeast cells. A preconcentration factor of 7 was achieved for As(III) when 35 ml of water were processed. The detection limits for As(III) and As(V) were 0.1 and 0.5 ng ml^-1, respectively. The speciation of inorganic As in different kinds of natural waters was performed following the proposed method. Spiked water samples were also analyzed and the recoveries in each phase were in all cases between 92 and 106%.