An agar culture method for determining the numbers of methanogenic bacteria in sludge was developed. Hydrogen and carbon dioxide were provided as substrate and palladium as a reduction catalyst. Sodium sulphide could also be used for reducing the medium. A methanogenic organism could be identified by correlating methane production with presence of a particular type of colony which was the most abundant. Fifteen strains which were pure cultured were all similar and were classified asMethanobacterium formicicum. The culture count for methane bacteria in sludge ranged between 105and 108 per ml. The rate of fermentation and the efficiency of conversion of substrate into cell material were studied. Presence of an acetate-fermenting methanogenic organism was established but no pure cultures could be obtained.Methanobacterium formicicumappeared to be the only methane producer growing in the dilution series used for counts.