Guppies (Poecilia reticulata) were exposed to a complex mixture of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in the presence and absence of sediment for 21 days. Since the guppies were separated from sediment to prevent uptake of sediment into the intestines, uptake was possible only from water. The accumulation kinetics, bioconcentration factors and biota-sediment accumulation factors were determined for 13 toxic PCDD and PCDF congeners. PCDDs and PCDFs were bioavailable to fish. In the presence of sediment, however, the accumulation was reduced compared to the accumulation in the system without sediment. Bioconcentration factors were reduced between 15% for 2,3,7,8-TCDF and 82% for OCDD by the presence of sediment. Reduction is probably caused by the introduction of organic matter in the water. Biota-sediment accumulation factors ranged from 0.155 for 2,3,7,8-TCDD to 0.003 for OCDD. The effects of sediment increased with increasing hydrophobicity of the compounds.