Halogenated coamendments enhanced dechlorination of 31 microM of spiked 1,2,3,4-tetrachlorodibenzo-p-dioxin (TeCDD) and 49 microM of spiked 1,2,3,4-tetrachlorodibenzofuran (TeCDF) in sediments from San Diego Bay (CA, USA) and Tuckerton (NJ, USA). Dechlorination of 1,2,3,4-TeCDD occurred to a greater extent under methanogenic than under sulfate-reducing conditions. The most effective stimulation of 1,2,3,4-TeCDD dechlorination occurred with coamendment of 25 microM of 1,2,3,4-tetrachlorobenzene (TeCB), 2,3,4,5-tetrachloroanisole (TeCA), 2,3,4,5-tetrachlorophenol, or 2',3',4'-trichloroacetophenone plus 500 microM lactate and 500 microM propionate as electron donors. The 1,2,3,4-TeCDD dechlorination was evident after three months and sequentially produced mainly 1,2,4-trichlorodibenzo-p-dioxin, 1,3-dichlorodibenzo-p-dioxin, and 2-monochlorodibenzo-p-dioxin (MCDD). Monobromophenols (2-bromo-, 3-bromo-, and 4-bromophenol), monochlorophenols (2-chloro-, 3-chloro-, and 4-chlorophenol), 2,3,5,6-tetrachlorobenzoate, or electron donors alone stimulated less 1,2,3,4-TeCDD dechlorination, with activity apparent only after six months. The 1,2,3,4-TeCDD dechlorination produced 50 mol % 2-MCDD after six months in sediments from the more contaminated Graving Dock and Paleta Creek sites in San Diego Bay. The 1,2,3,4-TeCDD dechlorination by sediments from the less contaminated Shelter Island site in San Diego Bay and in pristine Tuckerton sediments did not produce 2-MCDD. Dechlorination of 1,2,3,4-TeCDF to tri- and dichlorinated daughter products was significantly enhanced by TeCB and TeCA. These results suggest that halogenated aromatic compounds with structural similarity to 1,2,3,4-TeCDD/F stimulate bacteria with the ability to dechlorinate chlorinated dibenzo-p-dioxin and furans.