Thermophilic (75 degrees C), anaerobic biodegradation of chlorobenzoates was investigated using different inocula from geothermal and non-geothermal environments. Microbial dehalogenation of 3-chlorobenzoate (0.5 mmol l-1 was achieved by two mixed cultures growing anaerobically at 75 degrees C. One culture consisted of a facultative anaerobe and two obligate anaerobes, one of which was a methanogen, isolated from terrestrial sediments from hot springs in New Zealand. The other culture, derived from a non-geothermal environment, consisted of a Clostridium spp. and a non-spore-forming obligate anaerobe. No degradation of either 2-chlorobenzoate or 4-chlorobenzoate was achieved by these thermophilic cultures over the same time period. This is the first reported biotransformation of this chlorinated aromatic at a temperature of 75 degrees C.