In general, the toxicity of the chlorinated benzenes to freshwater organisms increases with increasing chlorination. Chlorobenzene is least toxic with 50 percent effect concentrations for Daphnia magna, goldfish, fathead minnows, guppy, and bluegill in the range of concentrations from 15,900 ug/l to 86,000 ug/l with the cladoceran being a little more resistant than the tested fish species. The 48-hour EC50s for Daphnia magna are (ug/l): chlorobenzene 86,000; 1,2,4-trichlorobenzene 50,200; 1,2,3,5- tetrachlorobenzene 9,710 and pentachlorobenzene 5,280. The 96-hour LC50s for bluegill exposed to chlorobenzene, 1,2,4-trichloro- benzene, 1,2,3,5-tetrachlorobenzene and pentachlorobenzene are 15,900; 3,360; 6,420 and 250 ug/l. The dichlorobenzenes, discussed in detail in another document, are slightly more toxic than chlorobenzene. Toxicity reaches its maximum with acute effect concentrations of pentachlorobenzene in the range of 250 ug/l for the bluegill (96-hour LC50) to 5,280 ug/l for Daphnia magna (48-hour EC50). Embryo-larval tests have been conducted with the fathead minnow, and the chronic values are 286 and 705 ug/l for 1,2,4-trichlorobenzene (two tests) and 318 ug/l for 1,2,3,4-tetrachlorobenzene. Acute-chronic ratios from fat- head minnow data were 6.4 for 1,2,4-trichlorobenzene and 3.4 for 1,2,3,4- tetrachlorobenzene. A freshwater algal species also was more sensitive to more highly chlorinated benzenes with 96-hour EC50 values for chlorophyll a in the range of 232,000 ug/l for chlorobenzene to 6,780 ug/l for penta- chlorobenzene. The bioconcentration of chlorinated benzenes also increased with increasing chlorination. (log Pow; monochlorobenzene 2.83; 1,2,4-tri- chlorobenzene 4.23; 1,2,4,5-tetrachlorobenzene 4.93; pentachlorobenzene 5.63 and hexachlorobenzene 6,43). The whole body bioconcentration factors increased from 182 for 1,2,4-trichlorobenzene to 22,000 for hexachloro- benzene. Acute lethal effects in midge, rainbow trout, fathead minnow, and bluegill were not observed at concentrations approximating the solubility of hexachlorobenzene. As with the freshwater toxicity tests with fish and invertebrate species, there was an increase in effects with the more highly chlorinated compounds with at least a one order of magnitude decrease in 96-hour LC50 values between chlorobenzene and pentachlorobenzene for the mysid shrimp (16,400 and 16 ug/l) and the sheepshead minnow (10,500 and 830 ug/l). Chronic values for the sheepshead minnow were 222 ug/l for 1,2,4-trichlorobenzene and 129 ug/l for 1,2,4,5-tetrachlorobenzene. A saltwater algal species was more resistant than the fish and invertebrate species, with 96-hour EC50 values for chlorophyll a in the range of 343,000 ug/l for chlorobenzene to 2,230 ug/l for pentachlorobenzene. Bionconcentration factors for hexa- chlorobenzene were as high as 23,000 for edible portions of the pinfish. The available data for chlorinated benzenes indicate that acute toxicity to freshwater aquatic life occurs at concentrations as low as 250 ug/l and would occur at lower concentrations among species that are more sensitive than those tested. No data are available concerning the chronic toxicity of the more toxic of the chlorinated benzenes to sensitive freshwater aquatic life but toxicity occurs at concentrations as low as 50 ug/l for fish species exposed for 7.5 days. The available data for chlorinated benzenes indicate that acute and chronic toxicity to saltwater aquatic life occur at concentrations as low as 160 and 129 ug/l, respectively, and would occur at lower concentrations among species that are more sensitive than those tested. For comparison purposes, two approaches were used to derive criterion levels for monochlorobenzene. Based on available toxicity data, for the protection of public health, the derived level is 488 ug/l. Using available organoleptic data, for controlling undesirable taste and odor quality of ambient water, the estimated level is 20 ug/l. It should be recognized that organoleptic data as a basis for establishing a water quality criteria have limitations and have no demonstrated relationship to potential adverse human health effects. Trichlorobenzenes: Due to the insufficiency in the available information for the trichlorobenzenes, a criterion cannot be derived at this time using the present guidelines. 1,2,4,5-tetrachlorobenzene: For the protection of human health from the toxic properties of 1,2,4,5-tetrachlorobenzene ingested through water and contaminated aquatic organisms, the ambient water criterion is determined to be 38 ug/l. For the protection of human health from the toxic properties of 1,2,4,5-tetrachlorobenzene ingested through contaminated aquatic organisms alone, the ambient water criterion is determined to be 48 ug/l. Pentachlorobenzene: For the protection of human health from the toxic properties of pentachlorobenzene ingested through water and contaminated aquatic organisms, the ambient water criterion is determined to be 74 ug/l. For the protection of human health from the toxic properties of pentachlorobenzene ingested through contaminated aquatic organisms alone, the ambient water criterion is determined to be 85 ug/l. Hexachlorobenzene: For the maximum protection of human health from the potential carcinogenic effects due to exposure of hexachlorobenzene through ingestion of contaminated water and contaminated aquatic organisms, the ambient water concentration should be zero based on the non-threshold assumption for this chemical. However, zero level may not be attainable at the present time. Therefore, the levels which may result in incremental increase of cancer risk over the lifetime are estimated at 10E-5, 10E-6, and 10E-7. (Shortened)