Young, basaltic ocean crust exposed near mid-ocean ridge spreading centers present a spatially extensive environment that may be exploited by epi- and endolithic microbes in the deep sea. Geochemical energy released during basalt alteration reactions can theoretically support chemosynthesis, contributing to a trophic base for the ocean crust biome. To examine associations between endolithic microorganisms and basalt alteration processes, we compare the phylogenetic diversity, abundance and community structure of bacteria existing in several young, seafloor lavas from the East Pacific Rise at approximately 9 degrees N that are variably affected by oxidative seawater alteration. The results of 16S rRNA gene analyses and real-time, quantitative polymerase chain reaction measurements show that the abundance of prokaryotic communities, dominated by the bacterial domain, positively correlates with the extent of rock alteration--the oldest, most altered basalt harbours the greatest microbial biomass. The bacterial community overlap, structure and species richness relative to alteration state is less explicit, but broadly corresponds to sample characteristics (type of alteration products and general alteration state). Phylogenetic analyses suggest that the basalt biome may contribute to the geochemical cycling of Fe, S, Mn, C and N in the deep sea.