This study provides a first description of the morphology of Blue Mats: sessile, colonial folliculinid ciliates (Folliculinopsis sp.) that create dense bright blue carpets in certain Juan de Fuca Ridge vent fields and at vents elsewhere. In one area of widespread venting, for example, Blue Mats occupied approximately 70% of the substratum. The ultrastructure of the Blue Mat ciliates was investigated in samples from Axial Volcano on the Juan de Fuca Ridge using conventional scanning electron microscopy and thin section transmission electron microscopy. These Folliculinopsis sp. ciliates secrete and dwell in tubes (loricae). The loricae were colonized by both coccoid and filamentous bacteria-like structures. Greater densities of coccoid- and short-rod-shaped bacteria were found between rows of cilia on the ciliate body (zooid) and especially on the peristomal lobes (arm-like extensions typical to folliculinid ciliates). A coccoid bacterial morphotype (within and independent of a vacuole) was located throughout the ciliate cytoplasm. Groups of this organism clustered within vacuoles were regularly distributed along the ciliate cortex. Electron dense, vacuole-bound features characterized by stacked membranous structures were also found within the ciliate cytoplasm. These results suggest the existence of at least an endosymbiosis between Folliculinopsis sp. ciliates and bacteria at hydrothermal vents. The chemolithoautotrophic nature of these symbiotic bacteria remains to be confirmed. To our knowledge, this is the first report of a protozoan–bacterial symbiosis at vents, as well as the first reported symbiosis in folliculinid ciliates.