Gases released from anaerobic wastewater treatment facilities contain considerable amts. of volatile Me and hydride derivs. of metals and metalloids, such as arsine (AsH3), monomethylarsine, dimethylarsine, trimethylarsine, trimethylbismuth (TMBi), elemental mercury (Hg0), trimethylstibine, dimethyltellurium, and tetramethyltin. Most of these compds. could be shown to be produced by pure cultures of microorganisms which are representatives of the anaerobic sewage sludge microflora, i.e., methanogenic archaea (Methanobacterium formicicum, Methanosarcina barkeri, Methanobacterium thermoautotrophicum), sulfate-reducing bacteria (Desulfovibrio vulgaris, D. gigas), and a peptolytic bacterium (Clostridium collagenovorans). Addnl., dimethylselenium and dimethyldiselenium could be detected in the headspace of most of the pure cultures. This is the first report of the prodn. of TMBi, stibine, monomethylstibine, and dimethylstibine by a pure culture of M. formicicum.