The binding and bioavailability of pyrene was studied in the laboratory in two humic fresh waters and in a reference water without dissolved organic material (DOM), measured as dissolved organic carbon (DOC). The uptake of pyrene by Daphnia magna in short-term (24 h) accumulation experiments was fitted to a first-order rate-kinetic equation to calculate simultaneous uptake and elimination rates. The partition coefficients of pyrene to DOC (KDOC) were 37.1 x 103 in Pielisjoki River (9.4 mg DOC L(-1)), and 34.9 x 103 in Lake Kontiolampi (17.4 mg DOC L(-1)) waters, indicating similar binding affinities of pyrene for both humic waters. The uptake clearance of pyrene (ku) in the DOC-rich Lake Kontiolampi water was 290.4 ml g wet weight(-1) h(-1), which was lower than those in Pielisjoki River and DOC-free waters (395.8 and 346.0 ml g wet weight(-1) h(-1), respectively). Pyrene elimination (ke) was in average 0.2 h(-1) with no significant differences between the waters. The observed bioconcentration factors of pyrene in Pielisjoki River and Lake Kontiolampi waters were 84% and 46% of that in the DOC-free reference water, respectively, thus indicating that a high concentration of DOC is needed to decrease the bioavailability of waterborne pyrene to D. magna.