Microbiological investigations of deep-sea sediments recovered from the Peru Margin during the ODP Leg 201 (Hole 1229A, 1–110 mbsf) demonstrated that microoganisms were a consistent component throughout the profile. Optimization of the dilution factor and DAPI-staining procedures for direct cell counts allowed the determination of the abundance of the entire microbial community, which was about 108 cells per g dry sediment. Microbial diversity in discrete samples taken from the 110-m profile was analysed using horseradish-peroxydase-rRNA-probes. In general, the majority of the detected cells belonged to the Eubacteria kingdom with a dominance of sulphate-reducing bacteria. The composition of the suflate-reducing community varied with depth. Desulfobacteriaceae were dominant in the uppermost sulphate-reducing zone and Desulfovibrionaceae at deeper depths in the upward diffusing sulphate-rich brines. Both sulphate-reducing groups were also detected in the methanogenic zone. Similarly, Archaea were detected throughout the profile, not only in the methanogenic zone but also in the upper and lower sulphate-reducing zones.