The focus of this project was to provide the first enumeration and characterization of viruses in oceanic crustal fluids. Samples were collected from CORK observatories 1362A and 1362B during an expedition to the Juan de Fuca Ridge Flank. Enumeration of microorganisms by epifluorescence microscopy revealed that concentrations of cells and viruses were on the order of 104 and 105 ml-1, respectively. These concentrations are about ten times lower than those in the bottom seawater at the CORK location. Electron microscopy of the microbial community showed the presence of various viral morphologies, several of which were similar to known archaea-infecting viruses in the groups Rudiviridae, Fuselloviridae, and Globuloviridae. The morphological evidence for the presence of archaeal viruses was supported by sequence analysis of a microbial metagenome (performed in collaboration with C-DEBI researchers). Of the identifiable virus hallmark genes mined from the metagenome, 10-40% were most similar to known archaea-infecting viruses. The composition of the viral assemblages was more similar to terrestrial hot springs than to other marine habitats that have been investigated so far, suggesting the importance of elevated temperature on shaping the community and thus the associated viral assemblage. The abundance of both viruses and cells in what appeared to be pristine deep subsurface fluid samples implies an active role for viruses in the basaltic ocean crust. This means that investigation of the multi-faceted symbiotic interactions between cells and viruses will be required to fully understand the physiology, ecology, and evolution of the microbial community inhabiting the oceanic crust.