Large sulfur-oxidizing bacteria in the family Beggiatoaceae are important players in the global sulfur cycle. This group contains members of the well-known genera Beggiatoa, Thioploca and Thiomargarita, but also recently identified and relatively unknown candidate taxa including “Candidatus Thiopilula spp.” and “Ca. Thiophysa spp.” We discovered a population of “Ca. Thiopilula spp.” colonizing cold seeps near Barbados at ~4.7 km water depth. The Barbados population consists of spherical cells that are morphologically similar to Thiomargarita spp., with elemental sulfur inclusions and a central vacuole, but have much smaller cell diameters (5 to 40 mM). We used microscopy and rRNA methods to describe the Barbados mat community, and applied metatranscriptomics to explore gene expression by the Barbados “Ca. Thiopilula” and associated microbial community when exposed to anoxic, sulfidic conditions.