Abstract Sediment toxicity tests with the midge, Chironomus riparius, were carried out in a natural river sediment spiked with either di-2-ethylhexyl phthalate (DEHP) or di-isodecyl phthalate (DIDP). The phthalates were each dosed at three levels in the sediment up to 10000 mg phthalate/ kg (dry weight basis) and the test organisms were introduced as larvae (< 24 hours post-hatch). No effects, in any of the treatments, were found on the development of the midge based on a comparison with the controls of the numbers and sex of the emerging adults. Sediment concentrations were measured at the start and finish of the study (28 day period) and good agreement with the nominal levels was found. Accumulation of the phthalates and any metabolites was determined on a sample of the emerging adults by measuring their radioactivity from the 14C-labelled test substances. A biota to sediment accumulation factor of 1.5 and 0.6 was found for DEHP and DIDP, respectively.