Microbially-mediated detoxification of five synthetic pyrethroid insecticides was achieved by a mixed culture growing anaerobically at 75 degrees C. These pyrethroids were hydrolysed to non-insecticidal products after a period of 4-8 weeks’ incubation. Of these five permethrin was the most readily transformed and Fastac the most recalcitrant. The mixed culture consisted of a facultative anaerobe and two other obligate anaerobes including a methanogen. This culture was derived from terrestrial sediments from hot-springs in New Zealand. This is the first reported microbial biotransformation of these xenobiotics at a temperature of 75 degrees C.