Guppies (Poecilia reticulata) were exposed to a complex mixture of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in water for 21 days. It is the first study in which accumulation data of 15 toxic dioxins and furans were quantified simultaneously in a continuous exposure experiment. First-order uptake rate constants (k1) and first-order elimination rate constants (k2) were calculated for the toxic, laterally substituted congeners. Bioconcentration factors (BCFs) were calculated from both the ratios of concentrations in fish and water and from the ratios of the rate constants. Dioxins and furans were selectively accumulated in fish tissue. The non-laterally substituted congeners were either not observed in fish tissue or showed lower BCF values than their laterally 2,3,7,8-substituted isomers. The log BCF values of laterally substituted congeners ranged from 3.90 ± 0.06 to 5.27 ± 0.07. BCF values were smaller than predicted by existing linear relationships with log Kow. Several phenomena were discussed with respect to the observed bioconcentration behaviour, such as limited bioavailability, reduced lipid solubility, reduced membrane permeability and biotransformation. Furthermore, the similarity between structure-activity relationships considering bioaccumulation in fishes and the Ah-affinity in mammals was discussed.