Dissolved methane is present at concentrations exceeding 10 mM in the pore waters of sulfidic, salt-brine-enriched sediments underlying chemosynthetic communities at the base of the Florida escarpment. Light hydrocarbon samples were obtained from brine seep sediments by means of an in situ probe and push cores deployed by the deep submersible Alvin. Pore-water methane had a δ13C value of -83.3 ±7.0 (Peedee belemnite, N = 17), contained <1.3% modern carbon, and was enriched over ethane concentrations by 103 to 105; these results all indicate a fossil, biogenic carbon source within the Florida platform. Methane-rich brine fluids arriving at seep sites are depleted in dissolved sulfate, although they have been diluted twenty-fold with sulfate-rich seawater during transit. It appears that sulfate reduction and methanogenesis are important processes within the platform.