HCBD, a colorless liquid with a faint turpentine-like odor, has a water solubility of 5 ug/ml at 20?C. It has a melting point of about -21?C, a vapor pressure of 22 mm Hg at 100?C, and a specific gravity of 1.675 (Hawley, 1977). Unlike most short chain halogenated aliphatics, hexachlorobutadiene has a low vapor pressure and, thus, may not volatilize rapidly from the aqueous environment to the atmosphere. There is a narrow range of LC50 values, for freshwater fish and invertebrate species and hexachlorobutadiene, from 90 (96 hour LC50 for goldfish) to 326 ug/l. The chronic value for the fathead minnow is 9.3 ug/l with an acute- chronic ratio of 11. There was a wide range of bioconcentration factors of 29 to 2,300 for a variety of organisms, (calculated log Pow = 3.74). As with the freshwater organisms, there is a narrow range of 96-hour-LC50 values for saltwater fish and invertebrate species of 59 to 557 ug/l with the invertebrate species being more sensitive. No data are available to estimate chronic toxicity. The available data for hexachlorobutadiene indicate that acute and chronic toxicity to freshwater aquatic life occur at concentrations as low as 90 and 9.3 ug/l, respectively, and would occur at lower concentrations among species that are more sensitive than those tested. The available data for hexachlorobutadiene indicate that acute toxicity to saltwater aquatic life occurs at concentrations as low as 32 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 hexachlorobutadiene to sensitive saltwater aquatic life. Human Health: For the maximum protection of human health from the potential carcinogenic effects due to exposure of hexachlorobutadiene 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 4.47 ug/l, 0.45 ug/l, and 0.045 ug/l, respectively. If the above estimates are made for consumption of aquatic organisms only, excluding consumption of water, the levels are 500 ug/l, 50 ug/l, and 5.0 ug/l, respectively.