This paper describes an investigation using biological‐iron removal for the elimination of arsenic (III). Groundwater was spiked with sodium arsenite and filtered through a pilot unit. As the water filtered through the sand, arsenic was retained on the iron oxides which were continuously produced by the bacteriological activity. Under specific aeration and pH conditions, both arsenic and iron were eliminated. This process can be used for the design of a full‐scale biological treatment plant.