Cable bacteria couple the oxidation of sulfide at depth in sediments with the reduction in oxygen, nitrate, or nitrite near the sediment surface, thereby preventing the release of toxic hydrogen sulfide to the overlying water. We show evidence for a diversity of cable bacteria in sediments from hypoxic and anoxic basins along the continental margin of California and Mexico. Cable bacteria activity in this setting is likely to be periodic and dependent on the supply of organic matter and/or oxygen.