Data are available estimating the acute toxicity of lindane to seven freshwater invertebrates and 15 fish species. Freshwater crustaceans (sowbug and scud) are the most sensitive invertebrate species tested, and cladocerans are the most resistant. The range of species mean acute values for invertebrate species is 10 to 676 ug/l. Among the fish species tested, brown trout is the most sensitive with an acute value of 2 ug/l; goldfish is least sensitive with a value of 141 ug/l. The Freshwater Final Acute Value for lindane is 2.0 ug/l. No acute data are available for other hexachlorocyclohexane isomers and freshwater animals. Acute toxicity data for lindane are available for eight saltwater invertebrate and 11 fish species. Acute values for invertebrate species range from 0.17 to 3,680 ug/l. Pink shrimp are the most sensitive species tested, and the polychaete, Neanthes arenaceodentata, is the least sensitive. Saltwater fish species tested have a wide range of sensitivity to lindane and are generally less sensitive than the invertebrate species; LC50 values range from 7.3 to 104 ug/l. The Saltwater Final Acute Value for lindane is 0.16 ug/l. Data are available for BHC with one saltwater invertebrate and one fish species and indicate that BHC is less toxic than lindane. Chronic values for lindane are available for three freshwater invertebrate species and range from 3.3 ug/l for the midge, Chironomus tentans, to 14.5 ug/l for Daphnia magna. A chronic value of 14.6 ug/l is available for the fathead minnow. Acute-chronic ratios range from 7.5 for fathead minnow to 63 for the midge, and the Freshwater Final Chronic Value for lindane is 0.080 ug/l. No chronic data are available for any other hexachlorocyclohexane isomers nor for any saltwater species. Acute tests with a freshwater alga and three different BHC isomers indicated that the alpha isomer is more toxic than are gamma (lindane) and beta. Both freshwater and saltwater algal species were much more resistant to hexachlorocyclohexane than were the invertebrate and fish species tested. Bioconcentration factors for lindane with a variety of freshwater fish species ranged from 35 to 486; bioconcentration factors for saltwater species ranged from 130 to 617. No useful FDA action level or result of a chronic feeding study with wildlife is available for calculation of a Final Residue Value. Lindane: For lindane the criterion to protect freshwater aquatic life as derived using the Guidelines is 0.080 ug/l as a 24-hour average and the concentrations should not exceed 2.0 ug/l at any time. For saltwater aquatic life the concentration of lindane should not exceed 0.16 ug/l at any time. No data are available concerning the chronic toxicity of lindane to sensitive saltwater aquatic life. BHC: The available data for a mixture of isomers of BHC indicate that acute toxicity to freshwater aquatic life occurs at concentrations as low as 100 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 a mixture of isomers of BHC to sensitive freshwater aquatic life. The available data for a mixture of isomers of BHC indicate that acute toxicity to saltwater aquatic life occurs at concentrations as low as 0.34 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 a mixture of isomers of BHC to sensitive saltwater aquatic life. Human Health: For the maximum protection of human health from the potential carcinogenic effects due to exposure of a-hexachlorocyclohexane through ingestion of contaminated water and contaminated aquatic organisms, the ambient water concentrations 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. The corresponding recommended criteria are 92 ng/l, 9.2 ng/l, and 0.92 ng/l, respectively. If the above estimates are made for consumption of aquatic organisms only, excluding consumption of water, the levels are 310 ng/l, 31.0 ng/l, and 3.10 ng/l, respectively. For the maximum protection of human health from the potential carcinogenic effects due to exposure of b-hexachlorocyclohexane through ingestion of contaminated water and contaminated aquatic organisms, the ambient water concentrations 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. The corresponding recommended criteria are 163 ng/l, 16.3 ng/l, and 1.63 ng/l, respectively. If the above estimates are made for consumption of aquatic organisms only, excluding consumption of water, the levels are 547 ng/l, 54.7 ng/l, and 54.7 ng/l, respectively. For the maximum protection of human health from the potential carcinogenic effects due to exposure of y-hexachlorocyclohexane through ingestion of contaminated water and contaminated aquatic organisms, the ambient water concentrations should be zero based on the non-threshold assumption for this chemical. However, zero levels 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. The corresponding recommended criteria are 186 ng/l, 18.6 ng/l, and 1.86 ng/l, respectively. If the above estimates are made for consumption of aquatic organisms only, excluding consumption of water, the levels are 625 ng/l, 62.5 ng/l, and 6.25 ng/l, respectively. For the maximum protection of human health from the potential carcinogenic effects due to exposure of technical-hexachlorocyclohexane through ingestion of contaminated water and contaminated aquatic organisms, the ambient water concentrations should be zero based on the non-threshold assumption for this chemical. However, zero levels 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, 10E-7. The corresponding recommended criteria are 123 ng/l, 12.3 ng/l, and 1.23 ng/l, respectively. If the above estimates are made for consumption of aquatic organisms only, excluding consumption of water, the levels are 414 ng/l, 41.4 ng/l, and 4.14 ng/l, respectively. Using the present guidelines, satisfactory criteria cannot be derived ate this time due to the insufficiency in the available data for delta- and sigma- hexachlorocyclohexane.