In acute toxicity tests, green algae Selenastrum capricornutum, diatoms Nitzschia palea, adult snails Physa gyrina, juvenile cladocerans Daphnia magna, larval midges Chironomus tentans, adult amphipods Gammarus minus, juvenile fathead minnows Pimephales promelas, and embryo-larva stages of rainbow trout Salmo gairdneri and largemouth bass Micropterus salmoides were exposed for 4 hours (algae), 48 hours (arthropods and snails), 96 hours (fathead minnows), 7 days (largemouth bass), and 27 days (rainbow trout) to two phenols (phenol and β-naphthol), two azaarenes (quinoline and acridine), and two polycyclic aromatic hydrocarbons (naphthalene and phenanthrene) present in coal-derived oils. Median lethal or median effective concentrations (LC50s or EC50s) ranged from 0.03 mg/liter for phenanthrene and rainbow trout to 286.54 mg/liter for phenol and the green alga. The rainbow trout embryo-larva assay was the most sensitive of the test systems to all the chemicals except quinoline. For this last compound, systems with juvenile fathead minnows and largemouth bass embryos were the most sensitive. As test systems, fish embryos and larvae were the most sensitive, juvenile fathead minnows and arthropods had intermediate sensitivity, and algae and snails were the most resistant to the test compounds under the test conditions. Within each chemical class (phenols, azaarenes, and polycyclic aromatic hydrocarbons), toxicity increased with increased ring number except for the reversed relationship with the azaarenes and fathead minnows. Thus, β-naphthol (two rings) was 2 to 45 times more toxic than phenol (one ring); acridine (three rings) was 7 to 27 times more toxic than quinoline (two rings); and phenanthrene (three rings) was 3 to 9 times more toxic than naphthalene (two rings). There was also a relationship between increases in toxicity and increases in the calculated octanol-water partition coefficients of the compounds.