Abstract: Samples of limestone soil collected from a small karst catchment affected by acid deposition in Southwest China were collected for analysis of total sulfur (TS), organic S, SO₄^2--S, total reduced inorganic sulfur (TRIS), and groups of sulfate-reducing bacteria (SRB) and their populations with the soil sulfur sequential extraction method and microbiological methods in an attempt to determine forms of sulfur and to characterize SRB distribution in the soil. It was found that organic sulfur was the major form of sulfur, while SO₄^2- was that of inorganic sulfur in the limestone soil. Bacteria of Desulfobulbus genus and Desulfovibrio-Desulfomicrobium group were detected in the limestone soil. The depth of the soil layer where SRB began to increase in population and TRIS in content corresponded well to that where SO₄^2--S began to decrease in content, which indicates that dissimilatory SO₄^2- reduction occurs in the limestone soil. The high pH and low clay content of the soil are adverse to SO₄^2- adsorption; therefore the remaining SO₄^2- after biological S retention is easily leached out of the limestone soil. SO₄^2- entering the soil with atmospheric deposition is retained mostly in the form of organic sulfur, of which mineralization releases large volumes of SO₄^2- into rivers and groundwater for a long period of time after annual sulfur deposition rate dropped by a large margin, thus affecting physic-chemical properties of the soils and chemical composition of the surrounding waterbodies. Therefore, more attention should be paid to such environmental responses in future.