Plating techniques for cultivation of methanogenic bacteria have been optimized for two members of the genus Methanococcus. Methanococcus maripaludis and Methanococcus voltae were cultivated on aerobically and anaerobically prepared agar media. Maintenance of O(2) levels below 5 mul/liter within an anaerobic glove box was necessary during plating and incubation for 90% recovery of plated cells. Under an atmosphere of H(2), CO(2), and H(2)S (79:20:1), 2 to 3 days of incubation at 37 degrees C were sufficient for the formation of visible colonies. The viability of plated cells was significantly affected by the growth phase of the culture, H(2)S concentration, and the volume of medium per plate. In addition, colony size of methanococci was affected by agar type, as well as by the concentrations of agar, H(2)S, and bicarbonate.