Abstract Carbon and sulfur isotope ratios have been determined for more than 200 samples of Precambrian graphitic and sulfidic metasediments from the Superior and Churchill Provinces of Canada. The sediments were deposited in small sedimentary basins related to various Canadian greenstone belts. The age of the Archean samples is approximately 2.7 Ga., the Proterozoic samples about 1.8 Ga., The Archean organic material shows C isotope values between −47 and −15%. vs. PDB. The Proterozoic metasediments show a smaller range, with δ13C between −30 and −17%., A few carbonate samples yielded C isotopic compositions between −5.9 and −4.7%., Associated iron sulfides have δ34S-values ranging from −6 to +8%., which are consistent with isotopic fractionation effects either by inorganic or bacterial reactions in the sedimentary sulfur cycle. The great variability of organic C isotopic composition, especially in the late Archean, although affected by extensive postdepositional alteration, might indicate differences in the environmental conditions and/or the organic content of these sedimentary basins. Some extremely 13C-depleted organic carbon values support a global occurrence of environments likely dominated by methanotrophic organisms at the end of the Archean.