Cold seeps in the Gulf of Mexico are often dominated by mussels of the genus Bathymodiolus that harbour symbiotic bacteria in their gills. In this study, we analysed symbiont diversity, abundance and metabolic potential in three mussel species from the northern Gulf of Mexico: Bathymodiolus heckerae from the West Florida Escarpment, Bathymodiolus brooksi from Atwater Valley and Alaminos Canyon, and 'Bathymodiolus' childressi, which co-occurs with B. brooksi in Alaminos Canyon. Comparative 16S rRNA sequence analysis confirmed a single methanotroph-related symbiont in 'B.' childressi and a dual symbiosis with a methanotroph- and thiotroph-related symbiont in B. brooksi. A previously unknown diversity of four co-occurring symbionts was discovered in B. heckerae: a methanotroph, two phylogenetically distinct thiotrophs and a methylotroph-related phylotype not previously described from any marine invertebrate symbiosis. A gene characteristic of methane-oxidzing bacteria, pmoA, was identified in all three mussel species confirming the methanotrophic potential of their symbionts. Stable isotope analyses of lipids and whole tissue also confirmed the importance of methanotrophy in the carbon nutrition of all of the mussels. Analyses of absolute and relative symbiont abundance in B. heckerae and B. brooksi using fluorescence in situ hybridization (FISH) and rRNA slot blot hybridization indicated a clear dominance of methanotrophic over thiotrophic symbionts in their gill tissues. A site-dependent variability in total symbiont abundance was observed in B. brooksi, with specimens from Alaminos Canyon harbouring much lower densities than those from Atwater Valley. This shows that symbiont abundance is not species-specific but can vary considerably between populations.