Nine marine methanogenic Methanococcoides strains, including the type strains of M. methylutens, M. burtonii and M. alaskense, were tested for the utilization of N-methylated glycines. Three strains (NM1, PM2 and MKM1) used glycine betaine (N,N,N-trimethylglycine) as a substrate for methanogenesis, partially demethylating it to N,N-dimethylglycine, whereas none of the strains used N,N-dimethylglycine or sarcosine (N-methylglycine). Growth rates and growth yields per mol of substrate with glycine betaine (3.96 g dw per mol) were similar to those with trimethylamine (4.11 g dw per mol). However, as glycine betaine is only partially demethylated, the yield per methyl group was significantly higher than with trimethylamine. If glycine betaine and trimethylamine are provided together, trimethylamine is demethylated to dimethyl- and methylamine with limited glycine betaine utilization. After trimethylamine is depleted, dimethylamine and glycine betaine are consumed rapidly, before methylamine. Glycine betaine extends the range of substrates that can be directly utilized by some methanogens allowing them to gain energy from this substrate without the need for syntrophic partners.