Toxicity values for 83 mono and disubstituted benzenes evaluated in the 6-h static population growth inhibition assay with the autotroph Chlorella vulgaris were taken from the literature. Each chemical was a prior assigned a mechanism of toxic action. A high quality hydrophobicity-dependent (log P) structure-toxicity relationship was developed for the baseline nonpolar narcosis mechanisms of action. Similarly, separate relationships were developed for polar narcotic anilines and polar narcotic phenols.