A study has been made of the distribution of the S isotopes in the Precambrian rocks of the Sudbury district [Ontario]. The S isotope ratios obtained for the Ni irruptive including samples of the Cu-Ni ores, associated norite, and micropegmatite are consistent with the conventional picture of a single sheetlike intrusive of magma that underwent some gravity-controlled process of differentiation. The S isotope ratios obtained from Cu-Ni ore and associated norite samples from the Stillwater igneous complex in Montana, parallel those obtained for the corresponding Sudbury samples. The Cu-Zn-Pb deposits found in faults between igneous and sedimentary rocks in the center of the Sudbury basin, support the view that these ores are hydrothermal deposits derived from the micropegmatite possibly during the last stages in the differentiation of the irruptive. The sedimentary rocks S. of the Sudbury basin are found to be very uniform in S isotope content and to have a S isotope ratio almost identical with that of meteorites (delta S (super 34'= 0). The sediments inside the basin, on the other hand, vary markedly in S isotope content and have isotope ratios which extend beyond the range usual for igneous rocks. These results are in agreement with other evidence that the ancient sediments inside and outside the basin were laid down under quite different conditions. The results suggest that the sediments inside the basin, in particular the graphite-rich slates, were formed in an environment in which extensive localized biological activity took place. The low C 13 content of these slates, delta C 13 = -36.9 permil with respect to standard limestone, also suggests life processes in Proterozoic times.