Sulfides from Cerro Negro, a mining district in central Chile with stratabound (manto-type) Cu deposits hosted by sedimentary, mainly volcanoclastic rocks show a range in theta(34)S from -21.2 to -15.6 parts per thousand. This range is in contrast with values around 0 parts per thousand, which are typical for similar deposits in volcanic sequences, probably due to genetic differences. The S isotope ratios obtained for the Cerro Negro deposits can be explained by ore formation at low-temperature, consistent with the hydrothermal alteration mineralogy of the host rocks, deposited in a lacustrine environment containing organic material. Presence of red beds in the sedimentary sequence hosting the deposits suggests that circulating solutions were oxidized, allowing transport of soluble Cu and Fe sulfates. interaction with organic material would then reduce the sulfates, with precipitation of sulfides with theta(34)S S in the range measured at Cerro Negro. Bacterial activity may have been a contributing factor, but the available data do not allow a conclusion. An ultimate magmatic origin for the S of the sulfates from which the sulfides were formed is indicated by of ca. +10 parts per thousand,for barite occurring in the upper levels of the mineralizaded bodies.