The aerobic and anaerobic metabolism of the isoprenoid alkene squalene was investigated in a new type of marine denitrifying bacterium, strain 2sq31, isolated from marine sediment. Strain 2sq31 was identified as a species of Marinobacter. Under denitrifying conditions, the strain efficiently degraded squalene; of 0.7 mmol added per liter of medium, 77% was degraded within 120 days under anoxic conditions with nitrate as electron acceptor. Tertiary diols and methyl ketones were identified as metabolites, and an anaerobic pathway was suggested to explain the formation of such compounds. The first step in anaerobic degradation of squalene by strain 2sq31 involves hydration of double bonds to tertiary alcohols. Under oxic conditions, the degradation of squalene by strain 2sq31 was rapid and involved oxidative splitting of the C-10/C-11 or C-14/C-15 double bonds, in addition to the pathways observed under denitrifying conditions.