The microbially mediated reductive dehalogenation of aromatic compounds is potentially important in removal of chlorinated aromatic compounds from the environment. Thermodynamic data are presented which show that the reductive dechlorination of 3-chlorobenzoate to benzoate is exergonic, which led to the hypothesis that reductive elimination of chlorine from 3-chlorobenzoate yields biologically useful energy. In the present paper this hypothesis is tested. Experimental data were obtained with a defined 3-chlorobenzoate degrading methanogenic consortium. These data showed that (i) the molar growth yield of a defined 3-chlorobenzoate degrading consortium increased from 4.9 g protein per mol benzoate metabolized to 6.8 g protein per mol 3-chlorobenzoate when 3-chlorobenzoate replaced benzoate as energy source, and that (ii) the ATP level in starved consortium cells was twice as high when the cells were fed 3-chlorobenzoate than when fed benzoate. These observations show that the electrochemical potential between the redox partners of the H+/H2 (electron-donating) and 3-chlorobenzoate/benzoate (electron-accepting) couples is a potential source of energy and are consistent with the hypothesis that reductive dechlorination of aromatic compounds is coupled to a novel type of microbial chemotrophy.