Pure cultures of an aerobically grown Flavobacterium sp. were shown by hydride generation-cold trap-atomic absorption spectrometry to biomethylate inorganic antimony (III) supplied as potassium antimony tartrate. Growth inhibition of the Flavobacterium sp. by antimony (III) over the range 0-30 mg Sb l(-1) was assessed by optimising parameters within an extended logistic growth model. Antimony (III) concentrations over this range influenced both the extent of antimony biomethylation (up to 4.0 microg l(-1)) and the relative proportions of the involatile mono-, di, and trimethylantimony species formed. Provision of inorganic arsenic (III) alongside antimony (III) enhanced formation of the involatile methylantimony species up to eight-fold. The data are consistent with accumulation of involatile intermediates from an antimony or arsenic biomethylation pathway in culture supernatants. Low yields of methylantimony species (<0.03%) suggest that antimony biomethylation by the Flavobacterium sp. was a fortuitous rather than a primary resistance mechanism for this element. These findings demonstrate that anaerobiosis is not an obligate requirement for methylantimony formation in prokaryotes, thus broadening the range of habitats for potential formation of methylantimony species in nature.