The reductive transformation of pentachloronitrobenzene (PCNB), an organochlorine fungicide, was investigated with a mixed, methanogenic culture developed from a contaminated estuarine sediment. Batch assays performed with this enrichment culture resulted in the biotransformation of PCNB to pentachloroaniline (PICA), which was then sequentially dechlorinated as follows: PCA -> 2,3,4,5- and 2,3,5,6-tetrachloroaniline (TeCA) -> 2,4,5- and 2,3,5-trichloroaniline (TrCA) -> 2,4-, 2,5-, and 3,5-dichloroaniline (DCA) -> 3- and 4-chloroaniline (CA) (low levels). Glucose fermentation, methanogenesis, and dechlorination were not inhibited at an initial PCNB concentration up to 40 mu M, which is 27 times higher than its aqueous solubility. The addition of 25 mM 2-bromoethanesulfonate (BES) to the PCNB-amended culture resulted in the complete inhibition of methanogenesis, but the biotransformation of PCNB to PICA and its sequential dechlorination pathway were not affected. The addition of sodium azide(200 mg/L)to the PCNB-amended culture resulted in complete inhibition of methanogenesis, but did not inhibit the transformation of PCNB to PCA; however, PCA dechlorination was not observed, PCNB was also abiotically transformed to PICA in autoclaved culture media but at much lower rates as compared to the biotic assays. In contrast, the rate of PCNB to PICA transformation in autoclaved culture controls was similar to the rates observed in the azide-amended culture and the active enrichment culture, indicating that biotically derived reductants facilitated the observed transformation of PCNB to PCA. Dechlorination of PCA was not observed in any of the abiotic controls. These findings have significant environmental implications in terms of the fate and transport of PCNB, PCA, and its dechlorination products in subsurface systems.