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Abstract: In a microcosm study, the importance of different sources of organic matter (humic acids, sterile sediment, sediment, and a microbial extract) for the bioavailability of the hydrophobic pesticide chlorpyrifos to Chironomus riparius larvae was quantified. In the last two treatments biofilms were allowed to grow before 14C-chlorpyrifos addition. Chlorpyrifos accumulation was quantified after 25h of exposure and after 21h of depuration. Larval accumulation was twice as high in the microbial extract treatment (447±79μg/kgww larvae) and 1.7 times higher in the sediment treatment (371±33μg/kg). After depuration, chlorpyrifos accumulation in larval tissue showed even higher differences; 3.1 times higher tissue concentrations in the microbial extract treatment (218±21μg/kg) and 2.2 times higher in the sediment treatment (156±35μg/kg). In contrast, chlorpyrifos accumulation in the humic acid and sterile sediment did not differ from that in controls. These results show that living microbes and biofilms, by creating a microenvironment and providing food for larvae, markedly increase the bioavailability of chlorpyrifos to Chironomus riparius. [Copyright &y& Elsevier]