Isoprenoic compounds play a major part in the global carbon cycle. Biosynthesis and mineralization by aerobic bacteria have been intensively studied. This review describes our knowledge on the anaerobic metabolism of isoprenoids, mainly by denitrifying and fermentative bacteria. Nitrate-reducing beta-Proteobacteria were isolated on monoterpenes as sole carbon source and electron donor. Thauera spp. were obtained on the oxygen-containing monoterpenes linalool, menthol, and eucalyptol. Several strains of Alcaligenes defragrans were isolated on unsaturated monoterpenes as growth substrates. A novel denitrifying beta-Proteobacterium, strain 72Chol, mineralizes cholesterol completely to carbon dioxide. Physiological studies showed the presence of several oxidative pathways in these microorganisms. Investigations by organic geochemists indicate possible contributions of anaerobes to early diagenetic processes. One example, the formation of p-cymene from monoterpenes, could indeed be detected in methanogenic enrichment cultures. In man, cholic acid (CA) and chenodeoxycholic acid (CDCA), are synthesized in the liver from cholesterol. During their enterohepatic circulation, bile acids are biotransformed by the intestinal microflora into a variety of metabolites. Known bacterial biotranformations of conjugated bile acids include: deconjugation, oxidation of hydroxy groups at C-3, C-7 and C-12 with formation of oxo bile acids and reduction of these oxo groups to either alpha- or beta-configuration. Quantitatively, the most important bacterial biotransformation is the 7 alpha-dehydroxylation of CA and CDCA yielding deoxycholic acid and lithocholic acid, respectively. The 7 alpha-dehydroxylation of CA occurs via a novel six-step biochemical pathway. The genes encoding several enzymes that either transport bile acids or catalyze various reactions in the 7 alpha-dehydroxylation pathway of Eubacterium sp. strain VPI 12708 have been cloned, expressed in Escherichia coli, purified, and characterized.