Sediment cores from oxygen depleted zones of lake Léman (lake of Geneva) and from anoxic surface sediment from Voua de la Motte (France) were analyzed for their sterol compositions. 4- methylstanols were found to be major sterols in Motte lake sediment, which is rich in organic matter. In lake Léman, the relative abundances of the same 4-methylstanols increase with the depth in the core, but they remain always minor compounds. An extensive search for a possible external source for these compounds included the analysis of water, of the plankton and of the surrounding soil material. All these results, along with those of an in situ incubation experiment, give strong evidence in favor of their in situ bacterial biosynthesis.