We have developed a cost-effective and sensitive spectrophotometric method for the determination of arsenic at trace level using a new reagent, hexamethylene ammonium-hexamethylenedithiocarbamate (HMA-HMDTC). Here we show that arsenic reacts with HMA-HMDTC in acidic conditions to yield the As(HMDTC)(3) complex. We studied the Beer's law at 256 nm, which showed linearity over the concentration range 0.2-1.0 mu g/mL of arsenic. We have shown that molar absorptivity, Sandell's sensitivity and the detection limit of the method are 6.06 x 10(4) L/mol cm, 0.0012 mu g/cm(2) and 0.060 mu g/mL, respectively. We have applied this new method to the determination of arsenic in drinking water.