Pilot-plant tests of two treatment methods-activated alumina and ion exchange--for removing arsenic from drinking water were evaluated at the Fallon, Nev., Naval Air Station (NAS). The arsenic concentration was 0.080-0.116 mg/L, exceeding the 0.05-mg/L maximum contaminant level. Although the valence of arsenic was not determined, the prechlorination process and test results suggest it was probably arsenic V. Chlorinated drinking water from the NAS was used for evaluating the efficacy of treatment under several different conditions. The activated alumina and ion exchange systems were operated through three different loading and regeneration cycles each. The major water quality factors affecting the removal of arsenic by these methods were: pH of feedwater, arsenic concentration, sulfate concentration, and alkalinity.