Methods to stimulate vitamin B12 production in natural anaerobic consortia which in turn will stimulate degradation of halomethanes were explored. Two approaches were considered, one based on supplementing cultures with precursors of vitamin B12 biosynthesis and the second approach was based on supplying primary substrates which are known to induce vitamin B12 biosynthesis. Initial experiments evaluating vitamin B12 precursors were conducted in anaerobic batch assays with 0.5 g volatile suspended solids of methanogenic sludge, a mixture of volatile fatty acids, 100 μM carbon tetrachloride, and mineral medium. Only one of the precursors, porphobilinogen, significantly stimulated the reductive dechlorination. A second experiment was conducted evaluating a concentration series ranging from 40 to 900 μM porphobilinogen. As little as 40 μM of porphobilinogen had a significant stimulating effect on CT-dechlorination; however several hundred μM of porphobilinogen was required to achieve a similar effect as that obtained with 20 μM vitamin B12. Other assays were conducted evaluating the use of primary substrates which are known to stimulate vitamin B12 production such as 1,2-propanediol. Both propanediol and ethylene glycol were stimulatory to CT degradation compared to other substrates such as volatile fatty acid and methanol. This is an abstract of a paper presented at the proceedings of the 8th International In Situ and On-Site Bioremediation Symposium (Baltimore, MD 6/6-9/2005).