The accumulation kinetics of two waterborne polycyclic aromatic hydrocarbons (PAHs), benzo(a)pyrene (BAP) and phenanthrene (PHE), were studied in the mayfly nymph (Hexagenia limbata). The uptake clearance decreased while the bioconcentration of BAP increased with an increase in weight of the H. limbata nymph. The relationship between uptake clearance and bioconcentration for PHE was variable, and bioconcentration was greater for the heavier animals. Two kinetic models were used to evaluate the effect of nymph weight on disposition of PAHs: (a) the amount-uptake clearance model, similar to models most frequently used in environmental toxicology; and (b) a clearance-volume model, similar to models used in clinical pharmacology. The two models gave similar predictive results but were different in a few cases. These differences in common parameter estimation probably resulted from methodologies used and high data variability rather than the models themselves, since they are mathematically equal. Some of the parameters are unique to each of the models and are defined and described. The clearance of oxygen from water is inversely and linearly related to the weight of the mayfly nymphs, but oxygen clearances were always much less than the uptake clearances of the PAHs. The high PAH uptake clearance compared to oxygen clearance implies a greater surface area or efficiency for PAH accumulation from water.