In order to better assess the influence of organic matter on the bioavailability of hydrophobic organic contaminants, the effect of algae and POM of bacterial origin on the bioaccumulation of benzo[a]pyrene in Daphnia magna was evaluated. The bioaccumulation was monitored with increasing concentrations of particulate organic matter (POM) and dissolved organic matter (DOM).In all experiments, the presence of POM greatly reduced the bioaccumulation of benzo[a]pyrene. The reduction was more pronounced in the presence of algae, for which we observed a 99%-reduction effect in the presence of 6×105 cell/mL (equivalent to 5.3 mg C/L). The bioaccumulation of benzo[a]pyrene was decreased by 49% by organic matter of bacterial origin at 4.7 mg C/L.Assuming that benzo[a]pyrene was partitioned between water, DOM and POM and supposing that D. magna accumulated free benzo[a]pyrene via respiration and POM-bond benzo[a]pyrene via ingestion, bioaccumulation data allowed to estimate the dietary uptake rate of benzo[a]pyrene as well as partitioning coefficients KPOC and KDOC.Despite the ingestion of contaminated particles, we could not observe any dietary uptake of benzo[a]pyrene in daphnids. We verified, as usually supposed, that the bioaccumulation of benzo[a]pyrene to D. magna occurs mainly via direct contact. Very high partitioning coefficients (log KPOC between 5.2 and 6.2) were estimated. This study pointed out the great influence of biogenic organic matter on the fate and the bioavailability of benzo[a]pyrene in aquatic ecosystems.