Methanogens represented about 0.5% of the total bacteria in sediments from a Georgia salt marsh in which Spartina alterniflora is the predominant vegetation. The population of methanogens was composed of at least two groups of nearly equal size. One group was represented by cocci which were able to utilize trimethylamine and were unable to use H(2) or acetate. The second group was composed of two subgroups which were able to utilize H(2) but were unable to use trimethylamine or acetate. The more common subgroup included rod- or plate-shaped methanogens which could utilize isopropanol in addition to H(2) and formate. The second subgroup included Methanococcus maripaludis, which utilized only H(2) and formate. Other groups of methanogens were also present, including Methanosarcina sp. which utilized acetate, H(2), and methylamines. In addition to the overall variability in the types of methanogens, the numbers of methanogens in sediments also exhibited significant spatial variability both within and between tall- and short-Spartina zones.