A micro organism Lactic acid bacteria as an immobilized cell on a solid support was presented as a new biosorbent in a simple and sensitive spectrophotometric determination of As(III) in various samples using IH-benzo [d] imidazole-2-thiol (max 256 nm) at pH 3.0 -4.0 . Beer’s law was obeyed over the range of 0.01-0.1 μg L−1 . Here we show that arsenic reacts with IHbenzo[ d]imidazole-2-thiol in acidic conditions to yield the As- IH benzo[d]imidazole-2-thiol complex. We studied the Beer’s law at 256 nm, which showed linearity over the concentration range 0.2–1.0 μg /mL of arsenic. We have shown that molar absorptivity, Sandell’s sensitivity and the detection limit of the method are 6.06 × 104 L/mol cm, 0.0012 μg / cm2 and 0.060 μg / mL respectively. The detailed study of various interfering ions made the method more sensitive and selective. The recovery of As (III) from various samples range from 97.75 to 99.35%. The present method was successfully applied for the determination of As (III) in spiked, natural water samples