AbstractQuantitative structure–activity relationships (QSARs) with aromatic hydrocarbons were obtained. Biological response was measured by acute toxicity of several aquatic trophic levels. The chemicals assayed were benzene, toluene, ethylbenzene,o‐xylene,m‐xylene,p‐xylene, isopropylbenzene,n‐propylbenzene, and butylbenzene. Acute toxicity tests were carried out withScenedesmus quadricauda, as representative of primary producers;Daphnia spinulata, a zooplanctonic cladoceran;Hyalella curvispina,a benthic macroinvertebrate; andBryconamericus iheringii,an omnivorous native fish. The EC50or LC50was calculated from analytical determinations of aromatic hydrocarbons. Nonlinear regression analysis between the logarithm of the octanol–water partition coefficient (logKow) of each compounds and the toxicity end points was performed. QSARs were positively related to increases in logKowat all trophic levels. Intertaxonomic differences were found in comparisons of algae with animals and of invertebrates with vertebrates. We observed that these differences were not significant with a logKowhigher than 3 for all organisms. Aromatic hydrocarbons with logKowvalues of less than 3 showed different toxicity responses, with algae more resistant than fish and invertebrates. We concluded that this was a result of the narcotic mode of action related to liposolubility and the ability of the compound to reach its target site in the cell. The bioconcentration factor (BCF) achieved to start nonpolar narcosis fell almost 1 order of magnitude below the BCF expected from the logKow. Predicted critical body residues for nonpolar narcosis ranged between 2 and 1 mM. © 2006 Wiley Periodicals, Inc. Environ Toxicol 21: 118–124, 2006.