The aim of this study was to det. to what extent particle size dets. the occurrence of organometal(loid) compds. of the elements As, Sb, Sn and Te in freshwater sediments. In addn., the anthropogenic impact upon the distribution through differing usage of freshwater habitats was examd. vis-a-vis flowing water, fish farms and a maturation pond for the bio-treatment of wastewater. All habitats sampled were located in the basin of the river Ruhr, Germany. In addn. to the detection of high concns. of total metal(loid) content of As, Sb, Sn and Te in the maturation pond sediments, this habitat also possessed the highest concn. of organometal(loid) species. Interestingly, the concn. of monomethylated metal(loid)s was up to 100-fold higher than those of higher methylated species of the same element. A max. of 28 mg kg-1 MMAs, 18 mg kg-1 MMSb and 8 mg kg-1 MMSn per dry wt. was detected. A similar tendency was noted for all other freshwater habitats tested. In contrast to methylated arsenic (arsenic contg. pesticides are banned in Germany) and antimony species, there is no doubt that the alkyltin species detected, e.g. MBSn and DBSn, are of anthropogenic origin since biogenesis of these species does not occur. Alkyltins are, however, known to enter the environment in a continuous and diffuse manner via discharge to sewage and air. In samples from the maturation pond concns. of up to 86 and 11 mg kg-1 per dry wt. were detected for MBSn and DBSn, resp. The detection of methylated arsenic and antimony species indicates that biotransformation of these elements is occurring in freshwater habitats. Irresp. of the usage and (anthropogenic) demands on the freshwater habitats tested, the highest concn. of organometal(loid) species was always detected in the sediment fractions that contained the highest concn. of humic substances and comprised up to 40% clays and silt particles (<63 mm). Owing to their high surface area to vol. ratios these particles possess a high binding capacity for metal(loid) ions and are attractive microhabitats for microorganisms. The resulting microcosmos therefore has a high potential for the biomethylation of metal(loid)s. At this point in time it is not fully clear whether the high concns. of organometal(loid) species detected in the clay/silt fraction are produced in situ by microbial biotransformation of bound metal(loid) ions or whether, as is the case for inorg. ions, the organometal(loid) species are translocated to this fraction and bound. Regardless of the mechanism, the accumulation of organometal(loid) species in clay/silt fraction means that these species are held in contact with the interstitial water and are therefore highly bioavailable, with potentially toxic consequences for aquatic organisms.