The oldest known case of chemosynthesis is suggested for Lower Ordovician ophicalcites in the southern Quebec Appalachians (Canada). These ophicalcites are the end product of tectonosedimentary and hydrothermal alteration of ophiolites on the deep sea floor. Peloidal mats (mean delta 18 O -4.6 per thousand ; mean delta 13 C -0.4 per thousand ) coat micritic sediments (mean delta 18 O -4.8 per thousand ; mean delta 13 C +0.1 per thousand ) and ophiolite clasts associated with hydrothermal, low-temperature ( nearly equal 80 degrees C) vent fracture-filling calcite cements (mean delta 18 O -13.5 per thousand ; mean delta 13 C -0.2 per thousand ). The mats are interpreted to be the fossil analogues of bacterial white slime around modern vents. The mats are locally overlain by botryoidal cements (mean delta 18 O -3.4 per thousand ; mean delta 13 C +0.9 per thousand ) of former high-magnesium calcite mineralogy. Precipitation of this cement was likely mediated by bacteria. Stable-isotope ratios of peloidal mats and botryoidal cements indicate that the exhalatives were free of hydrocarbons and H 2 S. Modern and ancient hydrothermal vents on ophiolite substrates commonly are typified by high-temperature H 2 S-rich fluids, sulfide mineralization, and a diverse chemosynthetic fauna. Carbonatized ophiolite successions, however, are significantly different in their vent setting, as indicated by this Ordovician occurrence and a previously described Cretaceous example. The Ordovician ophicalcite is devoid of mineralization and of macro-biological activity. The lack of corrosion of carbonates in the Ordovician ophicalcite suggests minimal rates of H 2 S venting, and pseudomorphs of sulfates, now calcite, are found in the sediment surrounding the vents. These observations suggest that the venting system of this Ordovician case was somewhat similar to H 2 S-poor white smokers. In the absence of hydrocarbon or sulfide emanations, thermophilic bacterial sulfate reduction likely was active around these Ordovician vents.