Methanobacterium strain M.o.H. is a strict, anaerobic methane-forming bacterium which obtains its energy from the oxidation of hydrogen; the electrons generated are passed to CO2 which is reduced to CH4. Inhibition of CH4 biosynthesis by alkyl halides was first documented by Bauchop1 who reported that the addition of carbon tetrachloride, chloroform or methylene chloride to rumen fluid inhibited CH4 production and increased H2 production. Rufener and Wolin2 reported similar findings in an artificial rumen system. Van Nevel3 described the inhibitory effects of chloral hydrate on CH4 formation in sheep rumen fluid incubated in vitro and in vivo. These observations were extended by Wood4 who described the inhibitory effects of chloroform, dichloromethane, methylene chloride and propyl iodide on CH4 formation in cell extracts of Methanobacterium omelianskii. Photolysis was observed to reverse inhibition. The site of action of these compounds has not been identified, but the reversal of inhibition by light was taken as evidence that the site is a B12 complex involved in the final transmethylation reactions which lead to CH4 synthesis4. The similarity between chloroform and the active centre of 1,1,1,trichloro-2,2 bis p-chlorophenyl ethane (DDT) raised the possibility that this pesticide might also inhibit methane biosynthesis.