The distribution of bacterial biomass and activity down to 80 m below the sea floor was investigated in sediments from the Peru margin, collected as part of the Ocean Drilling Programme, leg 112. Bacteria were present in all sediment depths sampled. Although direct bacterial counts decreased with depth there was no indication of a more rapid decline in the deeper layers and thus it is likely that bacteria should be present to much greater depths than those examined in this study. A significant number of the bacteria within the sediment were dividing and hence the bacteria were active and not just surviving. Bacteria were able to be cultured in laboratory media, from all depths, further confirming the viability of these organisms in situ. Bacterial sulphate reduction and methanogenesis were measured even in the deepest samples, although there were marked changes in rates with depth. There were also significant changes in the dominant populations of different types of viable bacteria and their associated activity with sediment depth. Although some of this succession in bacterial activity could be related to chemical changes in the pore water, there were some subtle changes in bacterial activity, possibly related to changes in the bioavailability of organic matter, which would not be anticipated from purely chemical data. Where numbers of viable bacteria and activity were decreasing a brine incursion, from below, dramatically stimulated further growth and activity, even in sediments approximately one million years old.