Anaerobic bacteria isolated from Kepone-contaminated sediment were screened for their ability to degrade Kepone under anaerobic conditions. The most extensive degradation was produced by denitrifying bacteria grown on benzoate-nitrate medium. In enrichment cultures, Kepone was transformed more extensively than by pure cultures isolated from the enrichments. A gram negative-faculative rod, called K bacterium, transformed 8.8% of the Kepone in a benzoate-nitrate medium in the presence of 2000 ug/ml potassium nitrate under anaerobic conditions. Kepone transformation by K bacterium increased to 21.1% when the potassium nitrate concentration in this medium was 500 ug/ml of medium. Although monohydro-Kepone and dihydro-Kepone were produced as products of the transformation, less than 20% of the transformed Kepone was recovered as these products. Both K bacterium and the enrichment culture transformed [¹⁴C]Kepone. No ¹⁴CO₂, or new radioactive insoluble or soluble products were detected in spent media. Products which were more polar than Kepone, but could not be identified, were observed in GLC chromatograms. K bacterium attached to the Kepone crystals and was pleomorphic during Kepone transformation. These data support the hypothesis that Kepone is transformed when it is used by certain anaerobes as an alternate electron acceptor.