<jats:title>Abstract</jats:title> <jats:p>The bioavailability of 14C-lambda-cyhalothrin and its bioconcentration by larvae of the aquatic insect Chironomus riparius was studied in laboratory sediment–water systems. Ten different sediments with a range of physico-chemical characteristics were investigated. The chemical was applied to sediment slurries and fourth instar C. riparius were exposed in the test system for 48 h. The distribution of the chemical between the sediment, water and organism phases was determined. In addition, the bioconcentration of 14C-lambda-cyhalothrin by C. riparius in a water-only test system was determined.</jats:p> <jats:p>In all the sediment–water test systems, the vast majority of the 14C-lambda-cyhalothrin (&gt;99%) was adsorbed to the sediment. Aqueous phase concentrations varied between the test systems, as did concentrations of the chemical in the organisms. However, bioconcentration factors based on the aqueous phase concentrations showed little difference between systems, ranging from 1300 to 3400 (mean 2300, coefficient of variation 25%). These values were very similar to the bioconcentration factor determined in water alone after 48 h (2000). Bioconcentration factors based on measured concentrations of extractable 14C-lambda-cyhalothrin in the sediment phase were much more variable, although they were always &lt;1, ranging from 0.11 to 0.84 (mean 0.39, coefficient of variation 61%). Sediment bioconcentration factors were inversely proportional to the measured sediment Kds, which ranged from 3290 to 22 100. That is, the higher the proportion of the chemical that was adsorbed, the lower the sediment bioconcentration factor.</jats:p> <jats:p>The results of the study supported equilibrium partitioning theory. In sediment–water systems, the lambda-cyhalothrin that was bioavailable was equivalent to the amount which was measured in the water phase.</jats:p>