The effect of iron reduction on the microbial reductive transformation of pentachloronitrobenzene (PCNB), an organochlorine fungicide. was investigated with a mixed, methanogenic culture enriched from a contaminated sediment. Fe(III)EDTA, Fe(III)citrate (completely bioavailable) and FeOOH (less bioavailable) were used as the iron source. PCNB was transformed to pentachloroaniline (PCA), but dechlorination of PCA and methanogenesis were not observed in cultures amended with Fe(III)EDTA until all of the added Fe(III) was reduced to Fe(II). Although PCA dechlorination did not take place, low rate methanogenesis was observed simultaneously with iron reduction in the culture amended with Fe(III)citrate. In contrast, both methanogenesis and PCA dechlorination took place at the same time with iron reduction in the same mixed, methanogenic culture amended with FeOOH, but at a lower rate compared to the Fe(III)-free control culture. Addition of anthraquinone 2,6-disulfonate (AQDS) to the culture amended with FeOOH resulted in a higher iron reduction rate, as compared to cultures devoid of AQDS, and a lower rate of both PCA dechlorination and methano enesis. Therefore, the reductive dechlorination of PCA is adversely impacted under conditions favoring high iron reduction rates. The interaction of these potentially competing processes (i.e. iron reduction, methanogenesis, and dechlorination) can significantly influence the environmental fate of PCNB and its transformation products, especially in soil and sediments. (c) 2007 Elsevier Masson SAS. All rights reserved.