Biomethylation can happen in geogenic as well as in anthropogenic systems, whenever the following four conditions are fulfilled: (i) an anaerobic atm. (at least on a microscale), reducing conditions and slightly acidic pH values in the hydrosphere. (ii) high concns. of metal(loid)s in easily accessible forms (e.g. as ions). (iii) the presence of microorganisms with biomethylation potential (bacteria, fungi). (iv) the presence of transferrable Me groups. By using hyphenated anal. techniques, dozens of organometal(loid) species could be detd. in landfill and sewage gases as well as polluted sediments and soils in concns. from the low ng/m3 (resp. Kg) to the mid mg/m3 (resp. Kg) range. Environmental gases are known to contain up to five species of As, one of Bi, one of Cd, three of Hg, two of I, five of Pb, two of Sb, three of Se, four of Sn, and two of Te (altogether 28 compds. of 10 elements) in addn. to volatile methylated silicones; waters and sediments in the environment show a similar variety of species (addnl. those of Ge). In the light of available toxicol. data our empirical anal. results suggest that certain environmental scenarios may be of potential toxicol. concern (resp. organometal(loid)s given in bracketts): River and harbor sediments (Sn, As, Hg), soil near ore deposits (Hg), gaseous emissions from industrial sludge fermenter (Te, As, Bi, Sb), geothermal water (As, Hg, Sn), sewage gas (As, Bi, Sb, Se, Te), waste gas (As, Sb, Sn), and leachates (As). In summary it can be stated that compared to the omnipresent biogenic background, metal(loid)org. emissions from solid waste and contaminated soils and sediments are at least two magnitudes higher, and require toxicol. evaluations in respect to the health of people working or living near these sites.