The fate of six different branched-chain fatty acids (BCFAs) in an anaerobic environment of a river sediment was studied in vitro by culturing enrichment consortia. The anaerobic consortium of BCFA-degrading genus degraded BCFAs with tertiary carbons through beta-oxidation, followed by methanogenesis by methane-producing anaerobic bacteria. The consortium could not degrade BCFAs with quaternary carbon. Degree of branching at the alpha or beta position along the carbon chain interfered with the beta-oxidation mechanisms of the branched-chain fatty acid.