Knowledge about methylated species of bismuth in environmental and biological media is very limited. The presence of volatile trimethylbismuthine has been unequivocally detected in landfill and sewage fermentation gases but the trace concentrations of methylated bismuth species reported in a few polluted soils and sediments probably require further confirmation. In contrast to arsenic and antimony, no methylated bismuth species have ever been found in surface waters and biota. Volatile monomethyl-, dimethyl- and trimethylbismuthine have been produced by some anaerobic bacteria and methanogenic archaea in laboratory culture experiments. Bismuth methylation differs significantly from the one of arsenic and antimony because no Bi(V) compound is known to be formed in biological and environmental media. Moreover, alkylbismuth compounds are rather instable due to the easy cleavage of the weak Bi-C bond.