Cold-water coral ecosystems are more widespread, diverse and productive than previously thought. However, little is known about the interaction of deep-water corals with microorganisms. To understand whether coral species have specific prokaryotic communities, it is necessary to assess the within and between colony variability. This was studied based on 16S rRNA gene and denaturing gradient gel electrophoresis (DGGE) for one of the main cold-water corals Madrepora oculata at Rockall Bank off the coast of Ireland. We successfully applied a rapid, non-toxic and inexpensive method for extracting DNA for 16S rRNA gene fingerprinting of marine prokaryotic communities based on a heat and salt lysis with simultaneous salt extraction (HEATSALT). The within and between colony variability of the community composition of bacteria associated to the mucus and ectodermal tissue of M. oculata was then evaluated using a 16S rRNA gene PCR and DGGE approach. Bacterial community composition (BCC) clearly differed between living coral and reference samples (dead coral and surrounding water; 80% dissimilarity). A large within (35-40% dissimilarity between polyps) and between colony variability (ca. 50% dissimilarity) of BCC was detected. We also found preliminary evidence that BCC differed between M. oculata and Lophelia pertusa. The high intraspecific variability found has consequences for selecting sampling strategies when assessing bacterial diversity and refines the question of controlling mechanisms of bacterial diversity on corals. Sequencing of DGGE bands showed that Spongiobacter type phylotypes (STP) dominated the DGGE bands. STP of M, oculata were grouped together and were different from those detected in other corals and sponges. In addition, the high sequence diversity of STP Suggests specific ecological roles and adaptations of this group in M oculata.