Chloroaniline-based compounds are widely used chemicals and important contaminants of aquatic and terrestrial environments. The authors have found that chloroanilines can be biologically dehalogenated in polluted aquifers when methanogenic, but not sulfate-reducing conditions prevail. The halogens are replaced by protons in a series of reductive steps catalyzed by microorganisms. The sequential release of halogens from the para and ortho position of 2,3,4,5-tetrachloroaniline (2,3,4,5-tetraCA) resulted in the formation of 2,3,5-trichloroaniline (2,3,5-triCA) and eventually 3,5-dichloroaniline (3,5-diCA). Similarly, when 3,4-diCA was used as a parent substrate, it was transformed to 3-chloro-aniline (3-CA). Metabolites and end products were identified by their chromatographic mobility and their mass spectral fragmentation pattern. The reaction helps suggest novel bioremediation approaches for aquifers and other environments contaminated with these chemicals. (Copyright (c) 1989 American Chemical Society.)