Sulfate reduction and methanogenesis are considered to be mutually exclusive microbial reactions in marine sediments. Typically, methane does not appear in significant concentrations in sediment pore waters until almost all dissolved sulfate has been reduced to sulfide. An exception to this commonly accepted pattern occurs in an approximately 500-meter thick sequence of Quaternary carbonates on the continental margin of the Great Australian Bight. An unusual combination of geochemical and sedimentological conditions leads to extensive simultaneous sulfate reduction and methane production throughout the 500-m interval. A probable explanation for the co-production of these reduced gases in this deeper biosphere is the presence of noncompetitive substrates for the two types of microbiota. ¿ 2001 American Geophysical Union