We have built a highly sensitive surface plasmon resonance (SPR) sensor to detect: arsenic (As) in groundwater. Using several thiol-containing organic compounds as sensor probes, we have been able to discriminate arsenic levels below and above the US EPA maximum containment level (0.010 mg/L =10 ppb, according to 2006 regulation), in drinking water. The SPR sensor is simple comparing to conventional spectroscopic techniques currently employed for As delection, and can be potentially used for As level screening in groundwater. (c) 2006 Elsevier B.V. All rights reserved.