Abstract: Microbial formation and transformation of organometallic and organometalloid compounds comprise significant components of biogeochemical cycles for the metals mercury, lead and tin and the metalloids arsenic, selenium, tellurium and germanium. Methylated derivatives of such elements can arise as a result of chemical and biological mechanisms and this frequently results in altered volatility, solubility, toxicity and mobility. The major microbial methylating agents are methylcobalamin (CH3CoB12), involved in the methylation of mercury, tin and lead, and S-adenosylmethionine (SAM), involved in the methylation of arsenic and selenium. Evidence for the methylation of other toxic metal(loid)s is sparse. Biomethylation may result in metal(loid) detoxification since methylated derivatives may be excreted readily from cells, are often volatile and may be less toxic, e.g. organoarsenicals. However, for mercury, low yields of methylated derivatives and the existence of more efficient resistance mechanisms, e.g. reduction of Hg2+ to Hg0, suggest a lower significance in detoxification. Bioalkylation has only been characterised in detail for arsenic. Microorganisms can accumulate organometal(loid)s, a phenomenon relevant to toxicant transfer to higher organisms. As well as bioaccumulation, many microorganisms are capable of the degradation and detoxification of organometal(loid) compounds by, e.g. demethylation and dealkylation. Several organometal(loid) transformations have potential for environmental bioremediation.