Alteration of basaltic glass through the entire volcanic sequence of the Troodos ophiolite is partly bio-mediated. The following observations support this conclusion: alteration textures resembling microbes in form and size; altered glass locally shows high carbon concentrations at the alteration front and organic remains. The absence of DNA suggests that the bio-alteration is a fossil process. Temperatures calculated from δ18O of carbonate, assumed to have been in equilibrium with seawater, yield 27–65°C, and δ18Osilicate versus H2O relationships indicate seawater alteration. These data are consistent with biogenic alteration during an early stage of ocean-floor alteration of the Troodos oceanic crust some 70–90 Ma ago.