A methanogenic bacterial consortium was obtained after inoculation of benzoate medium under N2/CO2 atmosphere with intertidal sediment. A hydrogen donating organotroph and Methanococcus mazei were isolated from this enrichment. H2-utilising sulphate reducing bacteria were isolated under H2/CO2 in the absence of organic electron donors. The Methanococcus was able to produce methane in yeast extract medium under N2/CO2 if the H2 donating organism was present, and sulphate reduction occurred if the hydrogen utilising sulphate reducing bacteria were grown with the H2 donating organism. The ability of the H2 utilising sulphate reducing bacteria to inhibit Methanococcus competitively was shown in clutures containing both of these H2 utilising bacteria.