Methanogenium frittonii Harris et al. 1996 is a later synonym of Methanoculleus thermophilus (Rivard and Smith 1982) Maestrojuan et al. 1990.

The 16S rRNA gene sequence of [Methanogenium] frittonii DSM 2832(T) was determined and was found to be 99.9 % similar to the sequence of Methanoculleus thermophilus DSM 2373(T). DNA-DNA hybridizations between both strains revealed 86 % DNA-DNA binding, indicating that both strains belong to the same species. The determination of the DNA G + C content of both type strains, DSM 2832(T) and DSM 2373(T), revealed values of 56.1 and 59.1 mol%, respectively. Based on the phenotypic and genotypic characteristics, it is proposed to unite the species [Methanogenium] frittonii and Methanoculleus thermophilus under the name Methanoculleus thermophilus, which is the earlier synonym and hence has priority. Emended descriptions of the species Methanoculleus thermophilus and the genus Methanogenium are also given.