Toluene, also referred to as toluol, methylbenzene, methacide, and phenyl- methane, is an aromatic hydrocarbon which is both volatile and flammable (40 FR 194). The molecular structure is distinguished from that of benzene by the substitution of a methyl group for one hydrogen atom. Toluene has the molecular formula C7H8, a molecular weight of 92.13 g, a boiling point of 110.625�C, a freezing point of -94.9�C, a density of 0.86694 at 20�C, a vapor pressure of 30 mm Hg at 26.03�C, a refractive index of 1.4893 at 24�C, and a log octanol/water partition coefficient of 2.69. Toluene is only slightly soluble in water, 534,8 +- 4.9 mg/l in freshwater and 379,3 +- 2.8 mg/l in seawater. It is miscible with alcohol, chloroform, ether, acetone, glacial acetic acid, carbon disulfide and other organic solvents. Although toluene is a volatile compound and has been shown to be readily transferred from water surfaces to the atmosphere under ideal conditions, its transport and persistence under environmental conditions is not well known. In the atmosphere, toluene is subject to photochemical degradation to benzaldehyde and traces of peroxybenzoyl nitrate. It is known also that toluene can re-enter the hydrosphere in rain. Five freshwater species have been acutely tested with toluene, and the cladoceran, Daphnia magna, was more resistant than four fish species. The 48-hour EC50 values for Daphnia were 60,000 and 313,000 ug/l. The range of 96-hour LC50 values for the goldfish, fathead minnow, guppy, and blue- gill is 12,700 to 59,300 ug/l. The EC50 values for two algal species were 245,000 ug/l and higher. No chronic tests have been conducted with toluene and freshwater species. There was a wide range of EC50 and LC50 values for saltwater species of 3,700 and ug/l for the bay shrimp to 1,050,000 ug/l for the Pacific oyster. The 96-hour LC50 value for the striped bass was 6,300 ug/l. An embryo larval test has been conducted for the sheepshead minnow and effects were observed at 7,700 ug/l but not at 3,200 ug/l. The acute-chronic ration for this species and kelp have been tested and effects were observed between 8,000 and greater than 433,000 ug/l. Studies with the grass shrimp resulted in no observed effect of salinity, temperature, or life stage on acute lethality. The available data for toluene indicate that acute toxicity to freshwater aquatic life occur at concentrations as low as 17,500 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 toluene to sensitive freshwater aquatic life. The available data for toluene indicate that acute and chronic toxicity to saltwater aquatic life occur at concentrations as low as 6,300 and 5,000 ug/l, respectively, and would occur at lower concentrations among species that are more sensitive than those tested. For the protection of human health from the toxic properties of toluene ingested through water and contaminated aquatic organisms, the ambient water criterion is determined to be 14.3 mg/l. For the protection of human health from the toxic properties of toluene ingested through contaminated aquatic organisms alone, the ambient water criterion is determined to be 424 mg/l.